JIPT: Add a small paragraph-splitting parser.
JIPT: Add a small paragraph-splitting parser.

Summary:
Uses simple-markdown to add a function to split a string into its
constituent paragraphs, and a function to join those paragraphs back
into a source string.

Depends on D21241

Test Plan: none

Reviewers: emily

Reviewed By: emily

Subscribers: jlfwong, kevinb

Differential Revision: https://phabricator.khanacademy.org/D21242

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