Add LaTeX citation handling to nbconvert

This PR adds the ability for nbconvert to manage LaTeX citations.  These are entered into the markdown cells using data attributes:

```html
<strong data-cite="granger">(Granger, 2013)</strong>
```

Which gets converted to the following in the LaTeX document: `\cite{granger`

We then run BibTeX.  The user also has to override the bibliography block of the nbconvert template. See:

https://github.com/ipython/nbconvert-examples

For a full example.