OK, I give up. This should be relatively simple, but nowhere on the web can I find specific instructions on how you add a plugin to tinymce with plone 5.x. It's just maddening. Anybody out there feel like exlaining it?
You can check this in the Plone documentation.http://docs.plone.org/develop/plone/forms/wysiwyg.html#tinymce-plugins
I hope it helps.