Introduce 'rss_tag_pre' action, which fires between the xml and rss tags in a feed. 

props solarissmoke, wonderboymusic, DrewAPicture.
fixes #19017.
Built from https://develop.svn.wordpress.org/trunk@29014


git-svn-id: http://core.svn.wordpress.org/trunk@28802 1a063a9b-81f0-0310-95a4-ce76da25c4cd