Add Currency#initialize taking named arguments for all properties

Author: Sija

README.md

@@ -101,7 +101,7 @@ To define a new `Money::Currency` use `Money::Currency.register` as shown
 below.
 
 ```crystal
-currency = Money::Currency.from_json({
+currency = Money::Currency.new(
   priority:            1,
   code:                "USD",
   iso_numeric:         840,
@@ -112,7 +112,7 @@ currency = Money::Currency.from_json({
   subunit_to_unit:     100,
   decimal_mark:        ".",
   thousands_separator: ","
-}.to_json)
+)
 
 Money::Currency.register(currency)
 ```

src/money/currency.cr

@@ -71,7 +71,7 @@ struct Money
       wrap?(value) || raise UnknownCurrencyError.new("Can't find currency: #{value}")
     end
 
-    def initialize(@code, @subunit_to_unit)
+    def initialize(*, @priority = nil, @iso_numeric = nil, @code, @name = nil, @symbol = nil, @alternate_symbols = nil, @subunit = nil, @subunit_to_unit, @symbol_first = nil, @html_entity = nil, @decimal_mark = nil, @thousands_separator = nil, @smallest_denomination = nil)
     end
 
     # Returns the relation between subunit and unit as a base 10 exponent.