GeoJSON on steroids (using geojson-vt)

Hallelujah, it works! The super-duper-awesome GeoJSON support in GL JS.

Fixes #464 (see comments for details)
Fixes #670
Fixes #694 

@jfirebaugh to review.
GeoJSONSource now reuses most of Source logic, including tile loading, — it's just a quick way to silence the error thrown by `TileCoord.url` which doesn't get `this.tiles`, since for GeoJSON, we don't care about any URLs – it's just passed as `null`.