Adding onSelectionChange

[YurkaninRyan]

Hi everyone! This PR definitely isn't ready to go in yet (tests, code style, and CLA all still need to be completed), but I wanted to get it up here to try and foster some discussion.

Currently, there is no way to listen to the selectionchange event unless you do it outside the React Event System. I was wondering if there was room to add that functionality.

I know that the current onSelect plugin is a bit different then the actual select event listener. And that it actually will try to use selectionchange if possible. I wasn't sure if this improvement would require a whole new type of event, or if we should just augment onSelect.

Thoughts?

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[gaearon]

Thanks for the PR! Given the community focus on the file size I don't see us adding more event polyfills in the future, even for useful events. Maybe if you expose it directly without a polyfill we could take it.