Introduce ReactivePropertyConvertible.
Introduce ReactivePropertyConvertible.

Summary: Similar to MotionObservableConvertible, this allows types to be used as though they were a ReactiveProperty.

Reviewers: O4 Material Apple platform reviewers, #material_motion, O2 Material Motion, appsforartists

Reviewed By: #material_motion, O2 Material Motion, appsforartists

Subscribers: appsforartists

Tags: #material_motion

Differential Revision: http://codereview.cc/D2572

material-motion/material-motion-swift,material-motion/material-motion-swift