Error signature refactoring (#6)

- Refactors to a `val, err := Get("ENV")` based signature
- Changes most functions to a `val, err` signature to match this
- Breaks code into functional files for sanity
- Provides `Get` alias `GetString` for convention
- Renames `GetSlice` to `GetStringSlice` and supports more types
- Adds support for multiple `default` slice values through `default=[one,two,three]`
  - Does not require the brackets if there is only 1 default item.

Author: syntaqx

README.md

@@ -4,7 +4,24 @@
 [![codecov](https://codecov.io/gh/syntaqx/env/graph/badge.svg?token=m4bBKy3UG3)](https://codecov.io/gh/syntaqx/env)
 [![Go Report Card](https://goreportcard.com/badge/github.com/syntaqx/env)](https://goreportcard.com/report/github.com/syntaqx/env)
 
-`env` is an environment variable utility package for Go.
+`env` is an environment variable utility package for Go. It provides simple
+functions to get and set environment variables, including support for
+unmarshalling environment variables into structs with support for nested
+structures, default values, and required fields.
+
+## Features
+
+- __Basic Get/Set__: Simple functions to get, set, and unset environment variables.
+- __Type Conversion__: Functions to get environment variables as different types (int, bool, float).
+- __Fallback Values__: Support for fallback values if an environment variable is not set.
+- __Unmarshal__: Load environment variables into structs using struct tags.
+- __Nested Structs__: Support for nested struct prefixes to group environment variables.
+
+## Installation
+
+```sh
+go get github.com/syntaqx/env
+```
 
 ## Usage
 
@@ -24,8 +41,12 @@ func main() {
 
     // Assuming the value of HOSTS is a comma-separated list of strings
     // Example: some-host:8000,another-host:8000
-    hosts := env.GetSliceWithFallback("HOSTS", []string{"fallback-host-1:8000", "fallback-host-2:8000"})
-    fmt.Printf("Hosts: %v\n", hosts)
+    hosts, err := env.GetStringSliceWithFallback("HOSTS", []string{"fallback-host-1:8000", "fallback-host-2:8000"})
+    if err != nil {
+        fmt.Printf("Error getting hosts: %v\n", err)
+    } else {
+        fmt.Printf("Hosts: %v\n", hosts)
+    }
 }
 ```
 
@@ -39,51 +60,63 @@ and `required` to enforce that an environment variable must be set.
 package main
 
 import (
-	"fmt"
-	"log"
+    "fmt"
+    "log"
 
-	"github.com/syntaqx/env"
+    "github.com/syntaqx/env"
 )
 
 type DatabaseConfig struct {
-	Host     string `env:"DATABASE_HOST,default=localhost"`
-	Port     int    `env:"DATABASE_PORT|DB_PORT,fallback=3306"`
-	Username string `env:"DATABASE_USERNAME,default=root"`
-	Password string `env:"DATABASE_PASSWORD,required"`
-	Database string `env:"DATABASE_NAME"`
+    Host     string `env:"DATABASE_HOST,default=localhost"`
+    Port     int    `env:"DATABASE_PORT|DB_PORT,fallback=3306"`
+    Username string `env:"DATABASE_USERNAME,default=root"`
+    Password string `env:"DATABASE_PASSWORD,required"`
+    Database string `env:"DATABASE_NAME"`
 }
 
 type Config struct {
-	Debug    bool           `env:"DEBUG"`
-	Port     string         `env:"PORT,default=8080"`
+    Debug    bool           `env:"DEBUG"`
+    Port     string         `env:"PORT,default=8080"`
     Database DatabaseConfig
 }
 
 func main() {
-	var cfg Config
-
-	// Set example environment variables
-	_ = env.Set("DEBUG", "true")
-	_ = env.Set("PORT", "9090")
-	_ = env.Set("DATABASE_HOST", "dbhost")
-	_ = env.Set("DATABASE_PORT", "5432")
-	_ = env.Set("DATABASE_USERNAME", "admin")
-	_ = env.Set("DATABASE_PASSWORD", "secret")
-	_ = env.Set("DATABASE_NAME", "mydb")
-
-	if err := env.Unmarshal(&cfg); err != nil {
-		log.Fatalf("Error unmarshalling config: %v", err)
-	}
-
-	fmt.Printf("Config: %+v\n", cfg)
+    var cfg Config
+
+    // Set example environment variables
+    _ = env.Set("DEBUG", "true")
+    _ = env.Set("PORT", "9090")
+    _ = env.Set("DATABASE_HOST", "dbhost")
+    _ = env.Set("DATABASE_PORT", "5432")
+    _ = env.Set("DATABASE_USERNAME", "admin")
+    _ = env.Set("DATABASE_PASSWORD", "secret")
+    _ = env.Set("DATABASE_NAME", "mydb")
+
+    if err := env.Unmarshal(&cfg); err != nil {
+        log.Fatalf("Error unmarshalling config: %v", err)
+    }
+
+    fmt.Printf("Config: %+v\n", cfg)
 }
 ```
 
 ### Nested Struct Prefixes
 
-You can use nested prefixes to group environment variables. For example, you can
+You can use nested prefixes to group environment variables. This allows you to
+reuse the same struct in multiple places without having to worry about
+conflicting environment variables.
+
 
 ```go
+package main
+
+import (
+    "fmt"
+    "log"
+
+    "github.com/syntaqx/env"
+)
+
 type DatabaseConfig struct {
     Host     string `env:"HOST,default=localhost"`
     Port     int    `env:"PORT,fallback=3306"`
@@ -93,20 +126,72 @@ type DatabaseConfig struct {
 }
 
 type Config struct {
-    Debug    bool           `env:"DEBUG"`
-    Port     string         `env:"PORT,default=8080"`
-    Database DatabaseConfig `env:"DATABASE"`
+    Debug         bool           `env:"DEBUG"`
+    Port          string         `env:"PORT,default=8080"`
+    ReadDatabase  DatabaseConfig `env:"READ_DATABASE"`
+    WriteDatabase DatabaseConfig `env:"WRITE_DATABASE"`
+}
+
+func main() {
+    var cfg Config
+
+    // Set example environment variables
+    _ = env.Set("DEBUG", "true")
+    _ = env.Set("PORT", "9090")
+    _ = env.Set("READ_DATABASE_HOST", "read-dbhost")
+    _ = env.Set("READ_DATABASE_PORT", "5432")
+    _ = env.Set("READ_DATABASE_USERNAME", "read-admin")
+    _ = env.Set("READ_DATABASE_PASSWORD", "read-secret")
+    _ = env.Set("READ_DATABASE_NAME", "read-mydb")
+    _ = env.Set("WRITE_DATABASE_HOST", "write-dbhost")
+    _ = env.Set("WRITE_DATABASE_PORT", "5432")
+    _ = env.Set("WRITE_DATABASE_USERNAME", "write-admin")
+    _ = env.Set("WRITE_DATABASE_PASSWORD", "write-secret")
+    _ = env.Set("WRITE_DATABASE_NAME", "write-mydb")
+
+    if err := env.Unmarshal(&cfg); err != nil {
+        log.Fatalf("Error unmarshalling config: %v", err)
+    }
+
+    fmt.Printf("Config: %+v\n", cfg)
+}
+```
+
+### Slice Types Defaults
+
+When using slice types, if you are declaring a single value as the default you
+can use the `default` tag as normal:
+
+```go
+type Config struct {
+	Hosts []string `env:"HOSTS,default=localhost"`
 }
 ```
 
-While will prefix the environment variables in the `DatabaseConfig` struct
-with `DATABASE_` as they are looked for. This allows you to reuse the same
-struct in multiple places without having to worry about conflicting environment
-variables.
+However if you want to declare multiple values as the default, you must enclose
+the values in square brackets:
 
 ```go
 type Config struct {
-    ReadDatabase DatabaseConfig  `env:"WRITE_DATABASE"`
-    WriteDatabase DatabaseConfig `env:"READ_DATABASE"`
+	Hosts []string `env:"HOSTS,default=[localhost,localhost2]
 }
 ```
+
+This is necessary as the pacakge uses commas as a delimiter to split the struct
+tag options, and without the square brackets it would split the values into
+multiple tags.
+
+```go
+type Config struct {
+	Hosts []string `env:"HOSTS,default=[localhost,localhost2],required"
+}
+```
+
+## Contributing
+
+Feel free to open issues or contribute to the project. Contributions are always
+welcome!
+
+## License
+
+This project is licensed under the MIT license.