Filters the list of TinyMCE external plugins.
The filter takes an associative array of external plugins for TinyMCE in the form 'plugin_name' => 'url'.
The url should be absolute, and should include the js filename to be loaded. For example: 'myplugin' => 'http://mysite.com/wp-content/plugins/myfolder/mce_plugin.js'.
If the external plugin adds a button, it should be added with one of the 'mce_buttons' filters.