Customizer: Add meta capability `customize` which is mapped to `edit_theme_options`.

You can now allow users to access the Customizer, say for any user who can `edit_posts`, via `map_meta_cap` or `user_has_cap` filter. See ticket for examples.

props westonruter, nacin.
fixes #28605.
Built from https://develop.svn.wordpress.org/trunk@29170


git-svn-id: http://core.svn.wordpress.org/trunk@28954 1a063a9b-81f0-0310-95a4-ce76da25c4cd