Implement fast zoom on minus/plus keydown with shiftKey

Goal is to make it consistent with the zoomControl shift+click.

If you think it's a good suggestion, I'm still not sure:
- if we should also factorize the `shiftZoomFactor` with zoomControl
- if not, then if we need a `shiftZoomFactor` option at all

Thanks for reviewing :)
more compactly written as `((e.shiftKey ? 3 : 1) * this._zoomKeys[key])`