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:
* [ ] update docs
* [ ] merge https://github.com/mapbox/gist-map-browser/pull/1
* [ ] merge https://github.com/mapbox/github-file-browser/pull/12
* [ ] @tmcw review
Atlas doesn't use access tokens, so it's only relevant for geojson.io and I've set that token on the mapbox account.