Image for loaded data for 'post_select' field

[mralessio]

Started using Shortcake for my theme, Thank you for your plugin, it levels up a user expirience on the next level.
It'd be really great to get the thumbnail image when loading data for 'post_select' field. That would be very helpful especially in case of lots of posts. Select2 3.5 or 4 has built-in this feature.

Example:

[westonruter]

FYI: Inclusion of featured image in a post selector has been implemented in an Object Selector control for the Customizer.

[westonruter]

It's also now in a PR for Customize Posts: xwp/wp-customize-posts#196