allow labels to be placed across tile edges

- [x] add `symbol-avoid-edges` to spec https://github.com/mapbox/mapbox-gl-style-spec/pull/141

Cross tile labels will work the same way they do in mapnik maps. The vectortiles are padded with extra data that gets used to prevent most collisions. symbol-avoid-edges can be used to disable cross-tile labels in the cases this approach does not work well enough.

Labels that exceed tile boundaries may still get clipped during rendering because we're rendering tile-by-tile instead of layer-by-layer labels. #193 will fix this. Native already draws layer-by-layer.