Expose onClick callback for graphie.addMouseLayer

Summary:
Adds an onClick option for graphie.addMouseLayer. This will be used for
adding points at the mouse location in the perseus InteractiveGraph
widget.

Test Plan: Tested in new perseus interactive graph work

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4607