Fix return values print (#63)

[mcflugen]

@ewels What do you think of this as a fix to #63?

I think you probably want to run super().main (in RichCommand.main) with standalone_mode=False so that click doesn't exit or print error message. However, once BaseCommand.main has run and you have the return value, you don't need to pass it on unless RichCommand.main itself was called with standalone_mode set to False.

[ewels]

Fab, looks great - thanks!