First attempt at `\text` function

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