Improve help/usage text

[tukusejssirs]

I really miss the -h/--help option. You have already implemented -h for different purpose, so at least --help should be implemented.

Also, I think that usage output should be a separate function (e.g. usage()). For now, it could accept one parameter, either short (short usage help text when the program dies) or full (full help text when issued with --help (then it should return 0 as it is a success).

The full help text should include all options with there definitions (just like it is currently in the README.md.

The full help text should also include examples.