re #346: Match on \(...\) or \[...\] instead of $$...$$.

We use matched delimiters rather than the $$ start and end delimiters
expected by 'ckeditor_wiris'. Changed the plugin to search for matched
delimiters of either sort, and preserve the delimiters that were used
when saving the edited formula.

This is currently more than a little finicky with selections and such.
Need to investigate under what circumstances it's remembering a previous
selection and inserting equations in there.