Enhance MessageBus' `subscribe` with an option to replay most recent event

[pearofducks]

I've had several cases where the item being mounted could have raced with any publish events, so one needs to do a subscribe and a peek to ensure nothing was missed.

The immediate (or whatever name you prefer) flag would automatically do this peek action and replay it onto the callback.

Example:

Suggested:

messageBus.subscribe('channel', 'topic', () => {}, { immediate: true })

Current:

const cb = ({ payload }) => {}
mb.subscribe('channel', 'topic', cb)
const maybeEvent = mb.peek('channel', 'topic')
if (maybeEvent) cb(maybeEvent)