Theme Customizer: Use any image from media library for background image

[jjonesftw]

I suggest that users be able to use any image in the media library as a background image from the customizer. At this time, if a user changes the theme background from the customizer, he/she must upload the image. This is already available from the dashboard, under Appearance > Background - it gives the option "Or choose an image from your media library:" I suggest that this option available on the dashboard also be available on the customizer. A similar option should be available for header images.