action groups should be supported

[bgruening]

We should at least iterate over all action groups.

[hexylena]

Does this mean subparsers? E.g. python a.py foo --some --set --of --flags and python a.py bar --other --flags? I'm not seeing action groups reflected anywhere in docs.

[bernt-matthias]

I guess what @bgruening meant was https://docs.python.org/3/library/argparse.html#argument-groups (which I think should be rendered as sections) and maybe https://docs.python.org/3/library/argparse.html#mutual-exclusion (which might become conditionals).

[bernt-matthias]

I guess this would solve #20

[bernt-matthias]

Currently groups are just swallowed, i.e. no output is generated for arguments within groups.