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.
Shouldn't this have `autoplay` as an attribute as well?

Without the `autoplay` attribute, the video doesn't automatically play when the `videoid` changes. However it started playing after I've added `autoplay` here.