feat: make resources ES6 observables

[pemrouz]

The basic API simply streams change objects:

ripple.on('change', (name, change) => {})       // listen for all changes
ripple('resource').on('change', change => {})   // listen for changes to resource

Users should also be able to subscribe using the ES6 Observable syntax:

ripple.subscribe({ next, error, complete })
ripple('resource').subscribe({ next, error, complete })

for await (change of ripple) {}
for await (change of ripple('resource')) {}

I was thinking we could use the error handler if a stream throws, but it seems that according to this answer the observable wouldn't recover if error is called. Therefore, I don't think it should be used for application-level errors, but only when the stream is truly stopped (liked losing connection - although even that is recoverable 🤔).