Good day futtta,
your plugin works exactly as advertised. Now the only problem is: it conflicts with a newsletter plugin called Wysija. The main tool to use this plugin (the visual editor) is not usable (it depends on drag'n'drop: please see http://support.wysija.com/knowledgebase/the-visual-editor-isnt-working-properly/ ). When deactivating all plugins and activating them one by one yours unfortunately was the one to cause the conflict.
Please look into this and see if it can be fixed from your side. In doubt I will probably drop your plugin, but I would rather not, because it offers unique functionality other plugins don't offer.
Thanks in advance!