Make Vue-side state updates more obvious

[jamesnvc]

Right now, when the Vue view of the app state changes, it sends a pengine event called update, where the input state is the new value of state.

While this is okay for some simple cases (the demo "meals" app here just sanitizes the whole state and the todo demo just does nothing (thereby accepting the new state), it general we'd probably want to know what the old state was.

A simple solution might be to have the event be a functor, like update(OldState), or even something more specifically indicating exactly what values have changed...or perhaps the event should send the old state & the value in the update functor should be NewState?