Make it easier to develop local changes to js

`html.io` currently refers to a static cdn for the js, which is awkward for trying out changes to the js itself.

We have a hacky example of local dev of js here: https://colab.sandbox.google.com/gist/erikfrey/eb8b138dce06f7c2135e9d9690a09a8e/local_dev.ipynb#scrollTo=NaJDZqhCLovU

A proper solution might be to introduce an arg to html.render that takes in a different static path, and that starts up a temp webserver.