Add a Bouncy-Ball Demo using D3

[bryanbraun]

Add a bouncy-ball demo using D3.

This would be a good contribution for somebody familiar with D3 and its ecosystem.

I think the right supporting library to use would be d3-transition, but I'm not as familiar with that ecosystem, so hopefully you can confirm that via research or experience.

A few tips for making this:

1. Start by duplicating the /examples/template folder and renaming your copy to something like d3, and create the animation in /examples/d3/index.html
2. Look at the other demos for reference parameters. For example, you can find details on ball-size, bounce height, easings, cubic-bezier functions, and more.
3. To bring it into the main page, just add a new <input> and <label> for it in the selection bar.

[bryanbraun]

(update: there's a PR up to address this at #23)

[AmGarera]

Going to be submitting my pull tonight, got this done pretty quick

[bryanbraun]

Fixed in #26. Closing.