[wip] V5

Long-awaited rework of lots of things.

* Moves file input & output into modals
* Removes geocoding
* [ ] Adds address geocoding https://github.com/mapbox/geocode-many/issues/1
* [ ] Supports saving new files to github https://github.com/mapbox/github-file-browser/issues/11
* [ ] geojsonio.app for local file editing https://github.com/mapbox/geojson.io/issues/325