Active Markdown: an experiment

I'm considering actually using the Tangle controls for the interactivity. They do a really good job of being accessible and unobtrusive. Right now the controls are just vanilla jQuery UI. I started with native `<input type="range">` for simplicity, but switched to jQuery for quick compatibility.

Also, how does the notation feel? The goal is a balance between functional/useful and simplicity. Ideally, the plaintext version is just as understandable.