Change the image caption shortcode format to [caption ...]<a><img /></a>...

Change the image caption shortcode format to [caption ...]<a><img /></a> caption text + html[/caption]. That way HTML tags in captions are better supported and the shortcode wouldn't break when using the wrong quotes. Props  sushkov, nacin, fixes #18311

git-svn-id: https://core.svn.wordpress.org/trunk@20679 1a063a9b-81f0-0310-95a4-ce76da25c4cd