Object Rest/Spread Properties

This proposal was well received when presented to TC-39 and will continue to get work for ES7. Let's start experimenting with it.

https://gist.github.com/sebmarkbage/aa849c7973cb4452c547

@jeffmo @benjamn
Would also be good to add a test to assert that something like this doesn't work: `<div>stuff</div {...props}>`
And probably also that this doesn't work: `<div {...props}>stuff</div {...props}>`