Allow <front> path to add feed to front page only

This is a very handy module - thanks!

This simple patch adds the ability to use the special path '' to add a feed to only the front page of the site. You could also do this by configuring the feed to appear on the path that you have configured the front page to pull from (e.g. node, or a view), but it is better to use in my opinion, both because it is a Drupal convention used throughout core, and also means the feed will continue to appear even if you modify the configured front page path.