Convert fork_collab to use command-line args

@kelleydv I really appreciate the effort you put into #32, but Thor seems to make testing a lot easier/cleaner than Slop (not that either really have documentation around this :disappointed:).  Took a pass at converting a single action to using command-line arguments, which made it more manageable than tackling all at once.  Still needs:

* Testing of different auth methods
* Tighter constraints around command-line options
* More description/aliases around the different options (steal from [other PR](https://github.com/education/teachers_pet/pull/32/files#diff-bb1053cb2687c87b41a3ffa6f41564c8R109))

but hey, *not a regression* :wink: If this seems like a good-enough start, will merge and create issues for the rest.

/cc @johndbritton