Add choices to arguments and options

[nkpln]

Implements #89.

Add choices to arguments and options. The choices are listed in the help text in curly braces.
Example:

Description:
  Greets someone

Usage:
  greet [options] [--] <name>

Arguments:
  name                  Who do you want to greet? {Secrus, sdispater}

Options:
  -y, --yell=YELL       If set, the task will yell in uppercase letters [default: "a"] {a, b}
...

Please be aware that this is my very first PR and there might be some newbie mistakes. I very much appreciate any feedback.