override parse_known_args for completion to work with partial argparse tree

[dirk-thomas]

Fixes regression from #436.

[ivanpauno]

Now, verb autocompletion and completion of the following arguments work well.
But completion of the command stopped working (it do work on master).

e.g.:

ros2 no  # Did autocomplete well to `node` in master, not on this branch.
ros2 launch demo_ # Do autocomplete well to `demo_nodes_*` now, not on master. 
ros2 launch demo_nodes_cpp talker_  # Do autocomplete well now, not on master.

I'm not pretty sure of what's the problem (I would have to check argparse documentation).

[ivanpauno]

It definetly works and code looks fine, but I can't tell if there's a better way of doing this or not.
Adding a second reviewer sounds like a good idea, maybe @mjcarroll who reviewed #436.

[dirk-thomas]

I can't tell if there's a better way of doing this or not.

colcon is doing pretty much the same: https://github.com/colcon/colcon-argcomplete/blob/86206dc187b1c5a318439e7010789a8979e5d0f7/colcon_argcomplete/argument_parser/argcomplete/__init__.py#L72-L80

Therefore I will get this merged. If anyone has a better proposal we can always follow up with a separate PR.