[not ready] Offline

**usage:**
1. edit [config.json](https://github.com/mapbox/geojson.io/blob/offline/config.json) with tile endpoint
2. `make`
3. `serve -p 8080`

Support needs to be added to:
* [ ] [tokml](https://github.com/mapbox/tokml)
* [ ] [leaflet-geodesy](https://github.com/mapbox/leaflet-geodesy)
* [ ] [csv2geojson](https://github.com/mapbox/csv2geojson)
* [ ] [github-file-browser](https://github.com/mapbox/github-file-browser)


Still need to:
* toggle/remove: 
 * [x] github support
 * [x] share
* [ ] tests
* [ ] do something about raven/sentry
I was just following the pattern here: https://github.com/mapbox/mapbox.js/blob/mb-pages/Makefile#L22-L24, since we need to have the css local: https://github.com/mapbox/geojson.io/blob/offline/Makefile#L64-L66