Support URLS that have dropped "extend/"

git-svn-id: https://plugins.svn.wordpress.org/wporg-plugin-embed/trunk@833211 b8457f37-d9ea-0310-8a92-e5e31aec5664
Bump (1.3)

git-svn-id: https://plugins.svn.wordpress.org/wporg-plugin-embed/trunk@833212 b8457f37-d9ea-0310-8a92-e5e31aec5664