WordPress: Customizer Add-on Resources

  

https://github.com/iCaspar/multiple-select

http://justintadlock.com/archives/2015/05/26/multiple-checkbox-customizer-control

Making a custom control for the Theme Customizer

Multiple Select Lists in the Theme Customizer

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/