Add new shortcode_atts() parameter

As of WP 3.6 we'll be able to filter shortcodes using the new `shortcode_atts_{$shortcode}` filter.

This will be extremely useful in 3rd party plugins that want to alter or add to the shortcode atts array. One such example is my EDD featured downloads plugin. When I wrote it I had no choice but to create a whole new shortcode just to feature the downloads. Now it looks like I can just hook in and add what I need so the [downloads] shortcode can accept a new "featured" option.

Awesome filter :)

see: http://markjaquith.wordpress.com/2013/04/04/wordpress-36-shortcode-attribute-filter/