4.0.0

Breaking changes

• all classes have been renamed to lowercase to match the std convention
• part of the code has been moved to .cpp

Fixes and features

• allow an empty string to be valid positional arguments
• fix empty option name in option_error
• support parsing value from ENV variable
• allow '?' in the short option name
• ability to stop parsing on the first positional argument
• do not silently consume any option as a value of another option