Try porting to pandoc 1.12

I got started while upgrading my Haskell platform.
I followed [Pandoc's changelog](http://hackage.haskell.org/package/pandoc-1.13.1/changelog) to have an idea of how to proceed, but at least bibliography support is probably broken. I'm also not convinced that the "tex" format exists (there's no corresponding reader/writer), but I've left that in. (Why isn't there a datatype for formats? I see `String` is more extensible, but... well...)