[not ready] Offline

**usage:**
1. edit [settings.json](https://github.com/mapbox/geojson.io/blob/offline/settings.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
* [x] do something about raven/sentry