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
this should probably have a try/catch if we're json parsing, since an invalid value would cause the application to bail.