Perseus.fixupJSON magic method

Summary:
As talked about, add a method to pass json through an editor and get
out [a promise of] the actual json for that thing.

Also: I'm sorry

Test Plan: run Perseus.fixupJSON({}) in the js console

Reviewers: joel

Reviewed By: joel

Subscribers: justinjaffray

Differential Revision: http://phabricator.khanacademy.org/D8154