Add `interceptInputFocus` to the perseus API

Summary:
Allows clients to specify a function that gets called instead of
focusing an input.

Test Plan:
Added `interceptInputFocus` to test.html, tried clicking on and
tabbing to an input in test.html
Also tried with `interceptInputFocus` set to undefined/null and
make sure things still worked

I haven't really tested this enough; #yolo!

Reviewers: charlie

Subscribers: alex, laura, mgp

Differential Revision: http://phabricator.khanacademy.org/D9440