Support pointer events

[imaus10]

Pointer events are currently "unknown event handler properties" in React.

What is the current behavior?

This JS fiddle shows that attaching the onPointerDown event to a div produces this warning in the console:

Warning: Unknown event handler property `onPointerDown`. It will be ignored.
    in div (created by App)
    in App

What is the expected behavior?

Although pointer events are not supported by Apple, they are still useful to developers targeting other platforms. Perhaps a good middle ground would be to allow them, but print a console warning saying they may not work. Happy to put together a PR if someone points me in the right direction!

[jquense]

#12507 was recently merged!

[imaus10]

Awesome! Thanks.