Allows the usage of slide index in print mode

[foyarash]

Refers to #374

Allows to retrieve the slides index using the useDeck hook on print mode. Previously the value was always 0, because on the Print component, the slide index is not passed to the Slide component.

[alpinegizmo]

Even better would be to do this, which makes it possible to print the notes as well.

const Print = ({ slides, notes }) => {
  const outer = useDeck()
  const context = {
    ...outer,
    mode: modes.print,
  }

  return (
    <Context.Provider value={context}>
      {slides.map((slide, i) => (
        <Slide key={i} slide={slide} index={i} notes={notes} preview />
      ))}
    </Context.Provider>
  )
}

[maxammann]

Would be very cool to see this merged :)