bibtex2pandoc: patch from Nick Bart.

Proper treatment of language codes.
Patch from Nick Bart.

- Fixed typo.
- Fixed editortype routine which did not reliably set CSL editor
  variable if editortype was nonexistent. (If there's a simpler way,
  please fix.)
- doi, isbn, issn are shown uppercase in
  http://citationstyles.org/downloads/specification.html#appendix-iv-variables,
  but didn't work unless lowercase here.
- Decided to try concatenating school, institution, organization,
  howpublished, publisher (which all go to CSL "publisher"). Most don't
  co-occur, but a few do, and if so, no info will be lost. Comments on
  whether this works well in the wild are welcome.