Add functions to extract images from posts in various forms

props wonderboymusic. fixes #22960

git-svn-id: http://core.svn.wordpress.org/trunk@23772 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add functions to extract a URL from an arbitrary string, HTML, or the current $post

props wonderboymusic. see #23570

git-svn-id: http://core.svn.wordpress.org/trunk@23773 1a063a9b-81f0-0310-95a4-ce76da25c4cd