Allow retrieving comments by post type, status, author, author, name, or...

Allow retrieving comments by post type, status, author, author, name, or parent.  Fetch only published posts for recent comments widget. Props filosofo. fixes #16506 #12904

git-svn-id: https://core.svn.wordpress.org/trunk@17667 1a063a9b-81f0-0310-95a4-ce76da25c4cd