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.
Looks like this isn't a blacklist, but is implemented by [each element that doesn't allow Shadow DOM](https://code.google.com/p/chromium/codesearch#search/&q=areAuthorShadowsAllowed&sq=package:chromium&type=cs)