clean up warnings
Initial work on #7 (citation support).  Seems to work.

Still needs a BL option to turn it on and off, and some updated
documentation & pointers to relevant pandoc documentation: see
https://github.com/jgm/pandoc-citeproc and
http://johnmacfarlane.net/pandoc/demo/example9/pandocs-markdown.html (sections
YAML metadata block, Citations).