implements a unified command-line interface

[ddale]

This pull request implements a unified command-line interface similar to conda or git, and based heavily on the implementation in conda. To use:

$ hexrd help
$ hexrd gui
$ hexrd help indexing
$ hexrd indexing -f foo.yml
$ hexrd grains -h
$ hexrd grains -f -q foo.yml

[ddale]

I forgot to mention, as a result of this pull request, hexrd would become the sole entry point from the command line. I haven't converted makeMedianDark yet, are there any others?

Once this pull request is approved, I will also fix recent changes I made to the subpackage naming. I don't know why I made the recent change I made, probably the result of too little sleep. I think we decided on findorientations and fitgrains. I would change the cli accordingly:

$ hexrd find-orientations
$ hexrd fit-grains
$ hexrd make-dark

or perhaps even better:

$ hexrd find orientations
$ hexrd fit grains
$ hexrd make dark --method=median
$ hexrd make z-projection