Inter-widget communication layer

Summary:
Adds an `interWidgets` function for inter-widget communication.

This function is passed to each widget via props, and allows each
widget to inspect other widgets ids/info and obtain references to
them.

"Remember: abilities are not inherently good or evil, it's how you use
them." ~ Kyle Katarn

Please use this one with caution.

Test Plan: See passage refs, which use this.

Reviewers: michelle, charlie

Reviewed By: charlie

Subscribers: marcos, eater

Differential Revision: http://phabricator.khanacademy.org/D11886