Add \text{} i18n linting to lint_i18n_strings.py

Summary:
Add a filter for i18nizing MathJax \text{}s inside of exercises.
Because these are found inside javascript or inside <code> tags, they aren't
automatically translated as i18n strings, so they need to be manually
surrounded by $._ calls. This automates the process.

Test Plan:
Run lint_i18n_strings_test.py, see that it passes. Run
lint_i18n_strings on the exercises, see that it does something reasonable most
of the time.

Reviewers: csilvers, john

Reviewed By: john

CC: alpert, eater

Maniphest Tasks: T1030

Differential Revision: http://phabricator.khanacademy.org/D2423