Add embedded trinket to slide

@calebsmith thought I'd see what our new embeddable Trinkets look like in your reveal slides.  There are some formatting glitches at weird resolutions but otherwise look pretty slick!  I slapped one on a Lets Develop It slide in part 2.  The interactive examples aren't really the best choice for every code block (highlighted syntax works great for many of them) but seem cool for slides or exercises you'll spend more time on.

This PR isn't for merge, but I'd be interested to hear what you think if you get to try it out.  Playing around with the height and width of the iframe seemed to help formatting.

Hope all's well!