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
Oh man! How could we forget about google-youtube!? Should we build in custom thumbnail support to that element? https://github.com/GoogleWebComponents/google-youtube/issues/9