Introduce a new means of outputting a `<title>` tag in the theme head. Requires a theme to add support by calling `add_theme_support( 'title-tag' )`. This is the first step in adding a more robust means of generating and outputting the title tag.

See #18548.
Props obenland, chrisbliss18, joostdevalk.



git-svn-id: https://develop.svn.wordpress.org/trunk@30074 602fd350-edb4-49c9-b593-d223f7449a82