Where I've gotten with Customizer updates
As it turns out, shoving all of the post ids into one input element is much easier
Properly set `collection_item_ids` too
Legitimately functional code to do live previews of Collection Widget changes in Customizer
When the Customizer is performing a widget update, use the staged IDs in the form

The form re-renders in the Customizer and we want to make sure it has the appropriate values
Use our helper here too
Trigger after the iteration so all new values appear.
When the Customizer is saved, we want to save our collection items too

Unfortunately, this is the "best" way to do it - no real action for us to access
Merge pull request #4 from danielbachhuber/customizer-live-update

Live updates for Collection Widgets in the Customizer