[feature request]Add unselect event and return value

[happyshows]

Hi,

To improve the interactivity, can you add some event when use choose a list item and return to shiny?
Something similar to onChange, which is implemented.

Thanks.

[timelyportfolio]

Thanks so much for the idea. However, I am on a break. Also,jsoneditor is getting an entire rewrite josdejong/jsoneditor#337, so I'm not inclined to do much until it is ready.