Support TeX-in-<math>-tags, a la Wikipedia's math markdown

Created by: Quuxplusone

I don't trust myself never to write $$ or \[ by accident (in a non-math context) on my blog, so I'd like to see MathJax support Wikipedia-style markdown for math:

Inline as <math>1+2=3</math> or out-of-line as
<math mode=display>
    1+2 = 3
</math>

The docs for tex2jax indicate that tex2jax cannot handle delimiters with angle brackets in them, and I've verified that that's the case. However, mml2jax can handle <math> tags just fine, and I have managed to make a patch that implements the above-requested behavior.