Fix xylem --print-config

[NikolausDemmel]

From #17 (comment)

I'm not sure if this is desired, but I get this when using the --print-config global option:

It seems to be a limitation of argparse to not allow optional subparsers. The simplest fix might involve http://stackoverflow.com/a/22054398/1813258.

P.S.: I just stumbled upon http://docopt.org/ (the recorded talk is a nice watch). I seems like a really powerful and simple way to do CLI, but unfortunately it looks to be not as practical when options come from different places.

[NikolausDemmel]

Suggestion by @wjwwood: Do some manual parsing like in https://github.com/ament/ament_tools/blob/use_verb_pattern/ament_tools/commands/ament.py