Fix #7 ALT + Left/Right functionality not working on windows

Not preventing the defaultEvent on arrow keydown doesn't seem to
have a negative effect on smooth scrolling and should also fix any
other non-working default keyboard shortcuts composed by arrows keys.
When user is scrolling, prevent default action on modifier key press

This alows the alt key to be pressed and released on windows while
scrolling (in order to change the speed momentarily) without changing
focus to Google Chrome's menu.