added support for \cssId

Partially addresses issue https://github.com/Khan/KaTeX/issues/90 by adding ```\cssId{id}{math}``` command.

Examples:
- ```\cssId{id_1}\theta```
- ```\cssId{id_2}\frac{x}{y}```
- ```\frac{\cssId{denom}x}{\cssId{denom}y}```
- ```\cssId{first_sum}{1+2}+3``` (creates a ```<span>``` around ```1+2``` and produces the correct spacing)