Add hidden, experimental transformations widget

Summary:
Add hidden, experimental transformations widget

adds a new experimental widget that allows students to explore
geometric transformations

This is just a first pass, and you need to set
var ENABLE_TRANSFORMATION_EXPLORER = true
in src/editor.jsx to add the widget to the dropdown.
Note that the widget will render if you type
[[☃ transformation-explorer 1]] even when disabled.

This isn't ready to ship (by setting the flag = true) yet, but I think
given the time crunch eyes would be more useful earlier rather than
later

Note: this also includes the earlier commit: D4835

Test Plan: tested locally in test.html

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D4933