Refine command usage statements

[Octobug]

Is your enhancement request related to a problem? Please describe

Yes, though it's not a big one. Current command usage statements seem a bit irregular.

...
Available Commands:
  build       used to build answer with plugins
  check       checking the required environment
  completion  Generate the autocompletion script for the specified shell
  config      set some config to default value
  dump        back up data
  help        Help about any command
  i18n        overwrite i18n files
  init        init answer application
  plugin      prints all plugins packed in the binary
  run         Run the application
  upgrade     upgrade Answer version
...

Describe the solution you'd like

Well, I've modified a version with a PR(#1331) as below.

• There seems no open standard for command-line usage statements.
• We can see some "styles" from examples from the cobra docs.
  • Command's descriptions are capitalized
  • But flags' are not

$ go run cmd/answer/main.go --help
Answer is a minimalist open source Q&A community.
To run answer, use:
        - 'answer init' to initialize the required environment.
        - 'answer run' to launch application.

Usage:
  answer [command]

Available Commands:
  build       Build Answer with plugins
  check       Check the required environment
  completion  Generate the autocompletion script for the specified shell
  config      Set some config to default value
  dump        Back up data
  help        Help about any command
  i18n        Overwrite i18n files
  init        Initialize Answer
  plugin      Print all plugins packed in the binary
  run         Run Answer
  upgrade     Upgrade Answer

Flags:
  -C, --data-path string   data path, eg: -C ./data/ (default "/data/")
  -h, --help               help for answer
  -v, --version            version for answer

Use "answer [command] --help" for more information about a command.

Describe alternatives you've considered

A discussion might be needed.