Add support for org-bibtex

[bdarcus]

Just thought I'd add this here, based on this comment/conversation.

emacs-citar/citar#397 (comment)

And suggestions from @andras-simonyi:

Yes, ol-bibtex.el (née org-bibtex.el) also contains functionality to read BibTeX entries from org-bibtex files, and the most important function in this regard is org-bibtex-headline which reads a single BibTeX entry from the Org headline (properties) at point. A slightly modified form of that function (citeproc-bt-from-org-headline) is used in the citeproc-el org-bibtex reader -- the "parser" basically calls that function for all headlines in the Org file via org-map-entries. It'd probably be trivial to do something very similar in parsebib, although I'm not sure about the performance implications.

emacs-citar/citar#397 (comment)