Fixes T3183 - Disable tags

Summary:
This disables tags that users would have to bring in third party sources for.
In future we can re-enable with some careful CSP.
I added a disableTags option to slowparse so its easy to customize.

Test Plan:
- Run output_test
- Put <video></video> in webpage, see error

Reviewers: john

Subscribers: #cs

Differential Revision: http://phabricator.khanacademy.org/D13423