Added an option to render multiplication using \times.

Summary:
Elizabeth requested this for exercises covering early algebra.
The example in "Acceptable formats" is consistent with the sign used.

Test Plan:
Toggle between \cdot and \times on test.html
Confirm rendering of \times via testrender.html
Preview a scientific notation item on local copy.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D2721