Lemma, theorem, proof in markdown cell (notebook + latex pandoc nbconvert)

The idea is to render this markdown:
```
<div id=superresult42 class=theorem>
My super theorem
</div>
```

Into latex (pandoc) nbconvert:

```
\begin{theorem}
\label{the:superresult42}
My super theorem
\end{theorem}
```

Work also as a first step for lemma, and proof. 

So this pull request propose a mechanism in nbconvert to introduce pandoc filtering in a proper way, and implement CSS rules to be coherent with the notebook. The principle could be reuse for any fine control between notebook and nbconvert pandoc export format.