Summary:
Make all of the parsing functions keep track of whether they are
parsing in math mode or text mode. Then, add a separate lexing function to lex
text mode, which is different than the normal mode because it does weird things
with spacing and allows a different set of characters.
Test Plan:
- See that the normal tests work
- See that the huxley screenshot looks reasonable
- See that none of the other huxley screenshots changed
Reviewers: alpert
Reviewed By: alpert
Differential Revision: http://phabricator.khanacademy.org/D7578
