Experimental video description on click (works well on iPads)

Not a priority to merge, just a UI tweak based on #3. Can view the deployed behaviour here: http://haskell-lang.org/

I tested it on my iPad, too. I initially made it react to hover, but that makes no sense for touch screens.

Uses some JavaScript for now. I can make it use Fay but the pull request would be substantially bigger.

Ping @radix.