Customizable color samples

Instead of using a left border to show the corresponding color sample, use a `div` element.  This will allow everyone to shape it easily using CSS, leaving the JS logic alone.

The default shape is now a circle filled with `strokeColor` and bordered with the `fillColor`.  It could make more sense to use colors the other way around, but I think it's easier to read this way.

Added a few comments to `demo.css` to explain how to switch from circles to squares.