Support float values on priority property

[nvkelso]

Sister issue to tangrams/tangram#692 on JS side.

Currently the priority property of point and text draw styles only supports ints, but Tangram JS support floats. (Both support functions).

Tilezen is adding a new collision_rank property in v1.7 that we'd like to use along with min_zoom to return a float.

This comment is the tl;dr:

• Add label collision rank for pois and other layer tilezen/vector-datasource#988 (comment)

Specifically, this should allow a function like (which returns a float):

priority = feature.properties.min_zoom + (1 - 1 / feature.properties.collision_rank) * 0.1

[nvkelso]

/cc @tallytalwar