Take revision control out of the realm of a pure constant. Make it filterable.

* New filter: wp_revisions_to_keep

props ethitter, SergeyBiryukov. fixes #22289.

git-svn-id: http://core.svn.wordpress.org/trunk@23818 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Give themers tangible, user-friendly template functions to take full advantage of structured post formats.

* the_audio()
* the_video()
* the_image()
* get_the_media()

Also introduces:

* get_the_extra_content()
* the_extra_content()

Those two functions are like their non-extra versions, except that they
will have any post-format bits extracted. e.g. It's an image post, for
which the_image() will extract an <img /> tag. the_extra_content() will
output the content *without* that image.

props wonderboymusic. Herculean effort. fixes #23572

git-svn-id: http://core.svn.wordpress.org/trunk@23819 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: take full advantage of new structured post format template functions, see r23819.

Extract first video or image from Video and Image post format posts, and place the item above the title within its own HTML wrapper with `the_video()` and `the_image()'. Also, remove the item from the post content with `the_extra_content()`.

This allows us to meet the design requirements for Twenty Thirteen with minimal changes to the theme template files. Fixes #23620.

git-svn-id: http://core.svn.wordpress.org/trunk@23820 1a063a9b-81f0-0310-95a4-ce76da25c4cd