allow crosstile labels and add `symbol-avoid-edges`

This allows labels to cross tile boundaries if they are point labels and if `"symbol-avoid-edges": true` is not set. More details in https://github.com/mapbox/mapbox-gl-style-spec/pull/141.

in -js: https://github.com/mapbox/mapbox-gl-js/pull/688

@kkaefer 
To let the label be drawn outside of the tile boundaries. Cross-tile labels are [only added to the buffer in the tile the anchor is in](https://github.com/mapbox/mapbox-gl-native/blob/2ba4fe4bb9f9138c797f044fff6fb8cb2db198ee/src/renderer/symbol_bucket.cpp#L304) and then drawn without clipping. We could add the labels to both tiles' buffers, leave the clipping. Not clipping at tile edges looks slightly nicer when zooming out because the clipping is visible if the neighbouring tile hasn't loaded yet.