Added Mousescroll Option 

When scrolling with my laptop's touchpad, I noticed that too many lines were scrolled. This PR provides an option for how many lines are translated on each mouse-scroll. Normally, this option would be set to 3, but I saw aesthetic improvements when scrolling 1 line at a time so I made it the default.

This feature implemented by myself has been proposed in #185 & #273 but was denied in those instances because of (respectively) odd PR shenanigans and a lack of documentation. I hope this will be more to everyone's liking.

ex.
`:set mousescroll=3`
`:set mscr=1`