WordPress: Customizer Add-on Resources
admin
https://github.com/iCaspar/multiple-select
http://justintadlock.com/archives/2015/05/26/multiple-checkbox-customizer-control
How to Create a Multi-Category Control for the Theme Customizer
http://scottfennell.org/2015/05/28/adding-a-draggablesortable-multi-checkbox-control-to-the-wordpress-customizer/