Improve grouping of widgets into paragraphs

Summary:
With the addition of the paragraphFn option, we can hook into marked's
concept of a paragraph, which is better than the token approach we were
using before, which caused problems with **bold** and *italic* text.

The paragraph grouping is much better now:
{F14485}

vs:

{F14486}
before.

Test Plan:
* Open a perseus exercise, have a line similar to
  * "some **bold** text"
* ensure it all is on one line together

Reviewers: jack

Reviewed By: jack

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7458