ENH: Convert TeX inline math, $...$, into ReST :math:`...`

This is deliberately simplistic: no displaymath, only deal with expressions which fit in a single line.

The itch I'm scratching is the desire to be able to write things like

```
.. math::
     I_0^{(2)}(x) = ...

where $x$ is ... 
```
without having to type :math: for each one-letter variable.