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.
It doesn't actually display the page, it just causes the browser to fetch all of the resources required to render it. I think prefetch is the correct name, but the description may be lacking if that isn't obvious.