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.
The thinking behind the name was that `autoplay=1` won't actually break the embed on mobile browsers—the YouTube embed won't automatically play, but it won't get into a bad state. Calling `play()` will wedge the YouTube embed. So what I want to signify is whether it's safe to call `play()` or not.

And I went with `undefined` instead of `null` out of some vague concern for what the property value would be in the `<google-youtube playsupported="{{playsupported}}">` use case, where someone is just reading the attribute value into a bound template variable. But if the property value will be `null` in that scenario, I'll switch the assignment and logic elsewhere for `null`. (I should have just checked.)