Add section and code for curve area

@behdad

I wasn't 100% sure if I was putting my code in the right files, but this seems to work locally for me.

(Temporarily closed before some bureaucratic details can be worked out.)
As someone trying to math in the browser a fair bit, I really hate the `Math.` namespace, so all `Math.[...]` functions are aliased to their plain name up top. `if (abs(loopArea) > 1) {` will do, here.