added support for contenteditable

I'm not sure if you want to use this, but this is how I was able to two-way bind content editable spans
* only spans, i could probably modify it so that its opt-in meaning only if the page has content editable, will it make those observable
* innerHTML and innerText don't work, perhaps because down the pipe something is only handling the "value" key
* the samples are here for now https://github.com/cesine/frb-jsmtl/blob/master/README.md, but i can move them over later and turn them into actual specs if you want