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. It feels way better. 

[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?

It is an option called `editor.scrollSensitivity`. Not sure what the units should be. I have it as a percentage of the original value, so the default is 40. It depends on what this feels like on windows.

Refs #1906