Grosser color support for prettier results

Test Plan: `\orange{x}\blue{-4}`, `\orange{x-}\blue{-4}` look the same after desaturation as `x-4` and `x--4`. In addition, `\orange{x}\blue{{-4}}` looks appropriately atrocious (i.e., has no space whatsoever).

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D3050