Create call-to-action buttons in the WordPress WYSIWYG.
Syntax: '<a class="button" href="' + e.data.url + '">' + e.data.link_text + '</a>'
Note: The link will also add target="_blank" rel="noopener noreferrer" automatically to external URLs.
Credits to the following article and author for providing this plugin a base to work off of and improve upon: http://www.wpbeginner.com/wp-tutorials/how-to-create-a-wordpress-tinymce-plugin/