added support for \cssId

Redoing this pull request because the last one didn't have any unit tests.  This one does.  ^_^

This implements the ```\cssId``` command as requested in issue https://github.com/Khan/KaTeX/issues/90.

Example:
- ```\cssId{id1}{1} + 2``` will add ```id="id1"``` to the span containing the ```1``` with the correct spacing
- ```1+\cssId{frac1}\frac{1}{x}``` will add ```id="frac1"``` to the span containing the fraction