Fix displaying docstring in command --help

[lewelyn7]

Hi, thanks for sharing great project! When I was using your lib i noticed this bug: ArgumentParser object requires keyword argument description instead of help according to https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser Now after this fix, when you type <command_name> -h also docstring from command class is printed.

Later I also noticed that, when i replaced help kwarg with description kwarg the <app_name> --help broke. There wasn't info about commands. So, finally instead of replacing arguments I added description kwarg and now the docstring is visible in both: app help and command help