Allow configurable button sets.

Summary:
Four functions always available, with addons.

Issues remaining:
* I don't remember exactly what button sets we wanted.

Test Plan:
In Perseus test mode, repeatedly select and unselect button sets, verifying
that they do show up as expected. All of the buttons insert the symbol or
construct I would expect of them.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D10910