This adds three new functions for getting/displaying attachment captions:
* `wp_get_attachment_caption` - Retrieves a caption for a specific attachment.
* `get_the_post_thumbnail_caption()` - Returns the post thumbnail caption.
* `the_post_thumbnail_caption()` - Displays the post thumbnail caption.
These are helpful for displaying a caption associated with an image directly
in a template, rather than using the caption shortcode.
This also introduces two new filters:
* `wp_get_attachment_caption` - Filters the value of `wp_get_attachment_caption()`.
* `the_post_thumbnail_caption` - Filters the display of the post thumbnail caption.
`the_post_thumbnail_caption()` is automatically filtered by `wptexturize()`,
`convert_smilies()`, and `convert_chars()` in `wp-includes/default-filters.php`.
Props flixos90, joemcgill.
Fixes#12235.
git-svn-id: https://develop.svn.wordpress.org/trunk@37915 602fd350-edb4-49c9-b593-d223f7449a82
0b4798b2bc