Implement \textcolor

`\textcolor{color}{math}` will apply a color to the given math content.  `color` can be any valid CSS Color Value (see [SitePoint docs][] (recommended), [Mozilla docs][], or [W3C spec][]).

[SitePoint docs]: http://reference.sitepoint.com/css/colorvalues
[Mozilla docs]: https://developer.mozilla.org/en-US/docs/CSS/color_value#Values
[W3C spec]: http://dev.w3.org/csswg/css3-color/#colorunits