deltaZ and firefox issue

Hi @anvaka!

Awesome package, I use it in [pan-zoom](https://npmjs.org/package/pan-zoom), in [gl-waveform](https://github.com/audio-lab/gl-waveform) and [plot-grid](https://github.com/dfcreative/plot-grid) in particular.

One gotcha.
I used `wheelDeltaX` and `wheelDeltaY` events, but they seem to be not polyfilled, but present in chrome.
So firefox just errored. I started digging and first question - is [there](https://github.com/anvaka/wheel/blob/master/index.js#L62) a typo? Should it be `deltaY`?

Maybe we should polyfill `wheelDeltaX` and `wheelDeltaY`?

Also it seem to be inconsistend through chrome/firefox :(. Same issue as [mouse-wheel](https://github.com/mikolalysenko/mouse-wheel).

Thanks!

Start with index.js at the linked line around the wheelDeltaX and wheelDeltaY handling, then compare the event properties available in Firefox and Chrome. Use the related mouse-wheel issue as context and verify that the resulting behavior is consistent across both browsers.

Written by the indexing model from the issue text.