Add postmeta setting and base control

[westonruter]

A page template and featured image are both stored in postmeta. These should use a WP_Customize_Postmeta_Setting with unique UIs for a page dropdown and (re-used) image media selector control. These can serve as the basis for creating more complex controls that map to multiple postmeta, or a control which manipulates a non-scalar postmeta value (an array).