Move bog-citekey-format to top of customization
Rename bog-citekey-heading

New name, `bog-citekey-from-heading-title', provides a clearer
indication of what it is doing.
Reword bog-citekey-from-heading-title docstring
Use org-get-heading

Somehow I missd this one when I was searching through org.el.
Reposition misplaced test heading
ENH: Allow citekeys to be properties

Now citekeys can be taken either from properties or from headings. This
provides greater compatibility with org-bibtex.el.