Add a \color command for custom colors

Summary:
Keep track of the color inside the style now, and use that when we are
rendering things. Added a custom lexing mode to handle lexing colors correctly.
Prefixed the old katex colors (internally) with "katex-" to distinguish them
from CSS colors.

Test Plan:
 - Run the normal tests, see they work
 - Run the huxley tests, see that they didn't change except for the color one
   which looks right

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7763