amsthm support

It'd be nice if I could hack pandoc a bit to get it to do something like

``` latex
\begin{theorem}
    If $S$ is a convex set ...
\end{theorem}
```

translated to

``` html
<p class="theorem">
    If $S$ is a convex set ...
</p>
```

Then, all I need to do is add a `theorem.css` or some such to get properly formatted theorems.
