Ignore whitespace following `\text`

Summary:
In something like `\text {hello, world}` we want to ignore the first space but not the second. This suggests to me that perhaps we shouldn't be in text-lexing mode until we actually see the first curly brace, so this changes us to do that.

Note that this disallows doing unbraced text things now; it seemed nontrivial to lex a single text character while ignoring spaces. We should probably fix it sometime, but for now we'll fall back to MathJax anyway so I'm not super concerned.

We should also add a huxley test for this.

Test Plan: `\text {2x 2}` rendered `2x 2` in a non-italic font.

Reviewers: jack, emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D9376