add opacity prop type to color layout

Summary: color and opacity are paired and should be in the same layout style definition. this adds opacity to color

Reviewed By: andrewimm

Differential Revision: D5018424

fbshipit-source-id: 31d2d79

Initial commit

fbshipit-source-id: d824f6c0bc73943e6f652110a3eb550e2f0f0f46