Add support for Hyperapp demo

[shershen08]

I've recently added a single-spa-hyperapp plugin to enable Hyperapp-based apps to work with single-spa

Do you think it worth adding it to the demo ?

[frehner]

Would be interesting to see, sure!

And maybe add to single spa docs too?

[shershen08]

I will submit PR, just I bit later, wanted to make sure ppl used it and it works ok for them