Wrap paragraphs within a question into components

Summary:
Going to make it easier to do the highlighting stuff, this commit
doesn't introduce any new functionality, but will make future commits
much simpler.

Introduces a way for paragraphs and the item-renderer to be aware when a widget is interacted with.

Test Plan:
* Open a perseus question
* Should still work fine

Reviewers: alex, jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7355