Fix footnote problem with optional arguments to citation commands

Fix footnote problem with optional arguments to citation commands

Scot Beker writes:

> I cite my references in org like
> this.\autocite[231]{bibtexkey_2009}, where '231' is the
> relevant page number.  When exporting to LaTeX, Org thinks
> that the value in square brackets is a footnote number and
> produces a document with a "footnote definition not found:
> 231" error message at the bottom of the document.  My
> settings are below, and I've searched pretty hard through
> them to look for anything that might have caused it.  Any
> ideas?