reStructuredText citations.

```
% pandoc -f rst
Here is a citation reference: [CIT2002]_.

.. [CIT2002] This is the citation.  It's just like a footnote,
   except the label is textual.
^D
<p>Here is a citation reference: <a href="#CIT2002"
class="citation">[CIT2002]</a>.</p>
<div id="citations">
<dl>
<dt><span id="CIT2002" class="citation-label">CIT2002</span></dt>
<dd>
<p>This is the citation. It's just like a footnote, except the label is
textual.</p>
</dd>
</dl>
</div>
```
