Support for custom thumbnails

@ebidel @addyosmani @robdodson 

- Adds a `thumbnail` attribute that can be set to the URL of a custom thumbnail image to use.
- Added another couple of tests to cover custom thumbnail behavior.
- Hopefully cleans up some element sizing inconsistency, in that the YouTube `<iframe>` and the thumbnail `<img>` (if present) are styled with `width: 100%`/`height: 100%`, and the element's `height` and `width` attributes are used to size the parent container.

Closes #9