Add support for event callbacks

[codemeasandwich]

someting like:

  setup(attachStore){
    const onStoreChange = attachStore() // No get state Fn
    onIncoming(onStoreChange)
  }

const cbs = []

// Called from outside
function messageIn(message){
  cbs.forEach(cbOnChange => cbOnChange(message))
}

function onIncoming(cbOnChange){
  cbs.push(cbOnChange)
}