Make options.forms backwards/future compatible

Summary: Shouldn't change anything for existing uses, but allows forms to be passed in as an array

Test Plan:
(1) Test for existing exercise (with it as a string delimited by commas)
(2) Test for exercise with it as an array

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7847