Add REGEXP to meta queries.

"REGEXP is usually a terrible idea to use, but I see no reason to disallow it."

props wonderboymusic.
fixes #18736.


git-svn-id: http://develop.svn.wordpress.org/trunk@25525 602fd350-edb4-49c9-b593-d223f7449a82
Make it possible to filter comment screen by post type; no UI. props johnbillion, c3mdigital. fixes #18596.


git-svn-id: http://develop.svn.wordpress.org/trunk@25526 602fd350-edb4-49c9-b593-d223f7449a82
Allow the "Add New" URL on post edit screens to be filtered via `admin_url()`. props SergeyBiryukov, jeremyfelt, c3mdigital. fixes #18504.


git-svn-id: http://develop.svn.wordpress.org/trunk@25527 602fd350-edb4-49c9-b593-d223f7449a82