Configurable MapboxAPITile + Github endpoint

Configurable MapboxAPITile + Github endpoint

**usage:**
1. edit `settings.json` with tile endpoint + github parameters
2. `make`
3. `serve -p 8080`

Also fixes #385 and disables Github buttons for open + save if not authed with Github.

To do:
* [x] update docs
* [x] merge https://github.com/mapbox/gist-map-browser/pull/1
* [x] merge https://github.com/mapbox/github-file-browser/pull/12
* [x] @tmcw review