Extract chats as structured data.

* add_chat_detection_format() — to add a chat regex pattern
* get_content_chat() — to grab a chat from content
* get_the_chat() — grab the chat from the current (or passed) post
* the_chat() — output the chat in formatted HTML
* paginate_content() — puts the <!--nextpage--> splitting stuff into a function
* get_paged_content() — grabs a page of raw content, needed to paginate chats properly

see #23625. props wonderboymusic, lancewillett.

git-svn-id: http://core.svn.wordpress.org/trunk@23804 1a063a9b-81f0-0310-95a4-ce76da25c4cd