Slow the scroll speed

Folks have been complaining about the mousewheel scroll speed -- it's too fast. It is really really sensitive. Turns out the deltas from the mousewheel event are generally regarded as [too fast](https://github.com/brandonaaron/jquery-mousewheel/issues/36). 

In my tests, the browser scroll speed is somewhere between 30 and 40% of atom's scroll speed. So I've slowed it down by that much. But [MDN](https://developer.mozilla.org/en-US/docs/DOM/DOM_event_reference/mousewheel) says the values are different depending on system and attached mouse. 

Anyone have an actual windows machine they can test this out on? @kevinsawicki are you using a real windows machine?

I think this should be an option, and will make it so. Though not sure what the units should be. Thinking 0 - 100, with the default being 40. But it depends on what this feels like on windows.

Refs #1906