Experimenting with custom elements

Hi there 👋

I'm using Choo for a large app and I'm currently exploring how I can put custom elements to use. I have some nicely working hacks that I've put into a repo to play around with (tested with Safari and Chrome):

https://github.com/mantoni/choo-custom-elements

Some thoughts / questions:

- Can we make it somewhat less hacky (e.g. the `onrender` setter)?
- Is there an interest in finding an "official" way to do this? 👮☝️😄
- Would it make sense to create a base class or a choo extension to make this easy?