Add support for localized strings

[cquike]

This library output some strings to the console that are hardcoded in English and cannot be localized. It would be nice if these strings could be translated to other languages using for instance something like gettext.
Examples of those strings are:

• Description of default options ("Show help", "Show version", "Verbosity level") in file Command.php
• Help messages ("Comand ... version", "Usage:", "Legend...") in Command.php
• Error messages like 'Invalid value. Please try again!' in Interactor.php

[adhocore]

yea thanks. in general i am okay with that but i think it is not very important. also i don't know many languages out there.
with that said, if you could send a PR for fixing it in the simplest way possible, then that would be great

[adhocore]

fixed in #120

[cquike]

Thank you for adding this feature!