Update the way a state is loaded into an event-stream

[elpoelma]

At the moment, when creating an event-stream using createReadStream it may take an existing state as optional argument. The state object you need to pass to this function can't currently be a fully JSON-parsed state, but may only be partially parsed.
A better solution may be to either accept an unparsed state as a string, or a fully parsed state as an object.