Make adding GeoJSON easier (skipping style modifications)

[mourner]

Make it dead easy to add GeoJSON data to LLMR without forcing the user to set up buckets/structure/styles etc. Maybe set up default ones for cases when GeoJSON id is not specified.

[jfirebaugh]

Good use case to cover: GeoJSON under labels.

[jfirebaugh]

I'm pretty happy with the current gl-js API:

mapbox-gl-js/debug/site.js

Lines 15 to 28 in 00d12c5

	map.addSource('geojson', {
	"type": "geojson",
	"data": "/debug/route.json"
	});
	map.addLayer({
	"id": "route",
	"type": "line",
	"source": "geojson",
	"paint": {
	"line-color": "#EC8D8D",
	"line-width": "@motorway_width"
	}
	}, 'country_label_1');

.

The last thing I think we should do here is add a placeholder layer with id labels to all the example styles that doesn't render anything but can be used to do map.addLayer({...}, "labels").

[mourner]

Yeah, looks pretty good now.