Move demo page onto codesandbox (or similar) environment

[jorroll]

As an alternative solution to the issue expressed in #365, move the demo page over to codesandbox.io so potential users can more easily explore and hack on rrulejs.

Example: https://codesandbox.io/s/rrulejs-demo-fd897?fontsize=14&module=%2Fdemo.ts

I think (but am not sure) that codesandbox supports connecting to a github repo. If not, then the major downside of this approach is that maintenance of the demo would need to be handled outside this repo.