Configurable MapboxAPITile + Github endpoint

Configurable MapboxAPITile + Github endpoint

Also fixes #385 and disables Github buttons for open + save if not authed with Github.

To do:
* [ ] update docs
* [ ] merge https://github.com/mapbox/gist-map-browser/pull/1
* [ ] merge https://github.com/mapbox/github-file-browser/pull/12
* [ ] @tmcw review