Handle `#+LINK_CITATIONS` metadata in org-mode.
Handle `#+LINK_CITATIONS` metadata in org-mode.

The values `true`, `yes`, and `on` are recognized as true values
(case-insensitive).

Closes #159.

adunning/pandoc-citeproc,jgm/pandoc-citeproc,jkr/pandoc-citeproc