citeproc-js "feature complete"

How about that. Looking at the todo list for citeproc-js, it just
dawned on me that I seem to have covered all of the existing elements
in CSL. Might have missed something, let me know if anything needs to
be added.

From here out, it should mostly be about (debugging and) extensions to
CSL, and mods to the API to support Zotero.