zoom plugin should add it's own state to Presentation State

[futurist]

When zoom in/out, the presentation state should also changed, so below demo code should supposed to work:

// zoom in
Reveal.zoom( {x: 10, y: 10, scale: 2} );

// below should contain the zoom state
let state = Reveal.getState();

Reveal.zoom( {scale: 1} );

// This restores the saved state, zoom 2x again
Reveal.setState( state );

Also it's even better if we can have an event for zoom, so that postMessage can send it via wire.