Shortlinks: Instead of allowing shortlinks for all post types, allow post types to register support for shortlinks
e.g. the Comics Post Type included in Jetpack: http://plugins.trac.wordpress.org/browser/jetpack/tags/2.5/modules/custom-post-types/comics.php#L217
Fixes #1967


git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@796611 b8457f37-d9ea-0310-8a92-e5e31aec5664