Ignore empty image captions

[nichtich]

Empty image captions in Markdown seem to produce empty captions in other output formats too (tested with 1.9.3):

![](foo.png)

becomes

<div class="figure">
<img src="foo.png" /><p class="caption"></p>
</div>

\begin{figure}[htbp]
\centering
\includegraphics{foo.png}
\caption{}
\end{figure}

I don't think that empty captions are very useful. A more common use case are images without captions, which one cannot create with pandoc right now. I'd prefer to skip empty captions if possible in output formats by default or by a command line switch:

<div class="figure">
<img src="foo.png" />
</div>

\begin{figure}[htbp]
\centering
\includegraphics{foo.png}
\end{figure}