Create a snippet that demonstrates extending a native element (fixes #29)

The samples were trickier and less exciting than I thought they would be. There is a set of bugs (https://code.google.com/p/chromium/issues/detail?id=234020) in Chrome that mean that you can't extend the Shadow DOM of most of the interesting native elements.
Since the element is hidden, I'm not sure that it matters exactly where in the DOM it goes. I will change it if you disagree, though.