Parse git docs

[emintham]

This is potentially two separate parts:

1. Interface with GitHub's API to checkout code, check for outstanding MRs, checkout those branches, etc.

2. Parse the docs to get the published fields in public API docs in a format similar to docdiffer to figure out which ones need to be documented. Also need to figure out if the currently published docs are accurate.

The pertinent part is mostly in markdown format and there are a couple of libraries to parse this. A gotcha is in the mostly part-- the library needs to be able to handle slightly malformed markdown and ignore those parts. We'll want to get out the JSON response from the markdown and also the tables of params.

Here's a partial list of markdown libraries with their benchmarks: lepture/mistune#1

MVP here is to be
A. Able to parse a local version of the docs repo at its checked out branch to figure out what fields each endpoint is exposing, params, etc. in the same format as docdiffer.