- Removes mock based test that checks delegation, just check commands result
- Allow setting of defaults that propegate down the chain
- Move internal parser args to a _parse helper.
This class is intended to be used to contruct a tree of sub-commands. In this
intial version it mainly focuses on exposing `add_argument` to allow for nested
parsers. Next step is to start adding better `--help` that shows the entire tree
and hooking it up to select what code path to really run.