[added] destinationMargin$
[added] destinationMargin$

Summary: As explained in the comment, this allows authors to account for decorative overflow (like shadows) when determining how far an item should animate.

Reviewers: O2 Material Motion, O3 Material JavaScript platform reviewers, #material_motion, featherless

Reviewed By: O2 Material Motion, #material_motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D3361

material-motion/material-motion-js,material-motion/material-motion-js,material-motion/material-motion-js