Add a post_format_meta filter.

props alexkingorg. fixes #24162.

git-svn-id: http://core.svn.wordpress.org/trunk@24080 1a063a9b-81f0-0310-95a4-ce76da25c4cd
post-new.php?format=X will pre-fill the format, to allow for Post-Format-chosen-first editor jump-starting.

props kovshenin, wonderboymusic. fixes #24116.

git-svn-id: http://core.svn.wordpress.org/trunk@24081 1a063a9b-81f0-0310-95a4-ce76da25c4cd