I have some improvements...

 * Add PANDOC_EXTENSIONS configuration variable, allowing one to
   enable or disable Pandoc's markdown extensions individually.
 * Remove dependency on pypandoc.
 * Don't change the working directory.
 * More efficient metadata extraction.

The most important aspect of this is the `PANDOC_EXTENSIONS` config variable.  The only way to enable or disable extensions is to tack `+whatever` or `-whatever` on the end of the input format name; this can't be done with the existing `PANDOC_ARGS`.

Unfortunately, pypandoc doesn't support this, so I just took it out -- it really wasn't doing much for us anyway.

The other changes are just tweakage.  (Why did it seem necessary to change the working directory?  As far as I can tell it is completely unnecessary.)