Add vertical list mixin

[yodasw16]

Vertical list only exists to reset the Horizontal List pattern. It makes more sense to just add a mixin to Horizontal list that does the same thing. You'll need to document this under "Other uses" in the README.

[motionsuggests]

Couple questions:

1. Should vertical list be removed?
2. "just add a mixin to Horizontal list" - how would this be a mixin? It seems like maybe it should be a variable for screen width to collapse at. Or I'm way off on the use case.

[yodasw16]

I was actually just looking at this one and was asking myself very similar questions. Yes, the vertical list should be removed. There's another issue for that.

The use case is when you are using a horizontal list and want to make it vertical instead. Basically undoing the horizontal list. I was thinking a mixin so that you could just use it in whatever media query you need it at. Now I'm kind of thinking that vertical just doesn't need to exist. If you are using this class, you just get a horizontal list and that's it. I don't want to set a variable to have its behavior change.

[motionsuggests]

That makes sense.