Add option to log for including context.
Add option to log for including context.

Summary: E.g. `$.log("Some context")`

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

Reviewed By: #material_motion, O4 Material Apple platform reviewers, O2 Material Motion, markwei

Subscribers: markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D2459

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