add completions for rm, pip_rm and channel_rm

[cjdoris]

i.e. look up the list of packages or channels and use this as possible completions

[cjdoris]

The same for the add commands would be nice, but these lists might be hard to get.

Looks like the conda repodata is stored in CONDA_ROOT/pkgs/cache/*.json, so we could parse those.