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
Should `<div attr1="foo" {...otherProps} />` be valid?
(and can you add a test to assert either way?)