Add support for sub-commands

[xenomachina]

Sub commands (as used by git or p4, for example) would be nice:

myCommand --foo --bar subcommand --baz --quux

The arguments after the subcommand should be disjoint from those of the main command.

[darth30joker]

it'll be great if we can support command like myCommand subCommand --foo 1 --bar 2