simple-markdown: handle escaping better

Summary:
Previously, we had a list of characters that we would break
text on to check for possible other syntax, and a list of
characters that were valid to escape. With this change,
we break text on any symbol, and can escape any symbol.

Test Plan: `make test`

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12714