Added the playsupported attribute

@ebidel @addyosmani & co.:

There’s some logic to determine if we’re on a platform that supports programmatic playback, and reflect that in the attribute.

It's very hacky, but as far as I can tell, it does work, though false negatives are a possibility if the timing of the autoplayback gets thrown off by something.

This closes #15 and closes #16.
If we're sticking with `position:absolute`, could you add a brief comment that captures the above inline in case a contributor wonders why we aren't just `display:none`ing this in future?