Add fill-extrude-height, fill-extrude-base, and light root property

[lbud]

This

• adds fill-extrude-height and fill-extrude-base as fill layer paint properties
• adds a light root object with the following properties, for use in lighting extruded features:
  • anchor
  • color
  • direction
  • intensity

Part of https://github.com/mapbox/mapbox-gl-js/issues/684, result of discussion on https://github.com/mapbox/mapbox-gl-style-spec/issues/456.

cc @lucaswoj @jfirebaugh @mourner @mollymerp @scothis @nickidlugash

[lbud]

ok ready ready ready @1ec5 👍 ?