Render MathML-syntax formulas

[cben]

Recognize and render <math>...</math>. Bonus points for namespaced use:

<foo >
  ...
  <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mrow>...</m:mrow></m:math>
  ...
</foo>

I don't see any point in verifying the scoped namespace prefix, just assume any prefixed math is also MathML.

Should extend the overlay mode to syntax-highlight xml (plus closetag addon?)