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.
The condition should be something like `!this._isGeoJSON` then, otherwise this will be broken:

```
new Source({
  type: 'vector',
  tiles: ['http://example.com/{z}/{x}/{y}.png']
});
```