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.
I've seen references to `<video>` being a no-op on certain browsers if `display: none` is set. We normally tell YouTube developers that they need to position the embed offscreen rather than setting `display: none`.