Add pandoc-filters.rb

Implements a class Pandoc, which supports most of the
same features as pandocfilters.py

Going to be replaced with a slightly less obtruse version
which hooks in to Hashes later.
Various stuff to do with pandoc-filters.rb.

Not really of any use to anyone: brief flirtation with the idea of porting the pandocfilters.py framework to ruby.
pandoc-multibib

A sketch of a filter to add multiple-bibliography support. Need to add function to convert citations to raw text after they have been parsed by pandoc-citeproc, but this seems to be quite a lot of work. Really the sort of thing that should be integrated into pandoc-citeproc.