Add yt-toggle for Polycast embeds

Adds a check for any elements with a `.yt-toggle` class and swaps their content with an iframe video player when the user clicks. The `.yt-toggle` element must have a `data-yt` attribute with the id of the YouTube video you wish to embed.

I'm still waiting on the thumbnail from GDS so I used one of Eric's devbytes to demo. I'll update the PR with the final videos and thumbnails when they become available.

@ebidel @addyosmani PTAL
One drawback to this approach is that autoplaying the video won't increment the view count and the playback won't count towards the other relevant YouTube Analytics stats.

An alternative if you're happy with the YouTube thumbnail image is to forgo the custom `<img>` and use an embed with [`controls=2`](https://developers.google.com/youtube/player_parameters#controls) set. This should make the embed lighter-weight than a standard YouTube embed, but there's still going to be a performance penalty due to the overheard of an `<iframe>` vs. an `<img>` at page load.

(If you do go the swap-out-an-`<img>` route, how about replacing it with a [`<google-youtube>`](https://github.com/GoogleWebComponents/google-youtube) instead of a plain `<iframe>` embed?)