It's very often that someone will need to process values that were not
provided separately (eg. generating a complex default value like in V).
This commit allows the user to call the *_opt() functions instead of
forcing them to provide some magical default values.
* flag: Implement int_multi(), float_multi() and string_multi()
These 3 functions are useful when the application expects a flag to be
repeated more than once.
A sample would be:
v -cflags someflag -cflags secondflag
This feature exposes the raw array that the flag parser internally keeps
and allow the application to parse it however they want instead of
dropping everything except the first element.
* flag: Add documentation
Add comments describing what *_opt() and *_multi() do.
* flag: allow to check the count of the free args to produce an error
* flag: allow to define flags with abbreviation
flags could be defined to use a single char as abbreviation like
'--output' or '-o'
'--help' or '-h'
