Add PaperWM.spoon (#304)

This is a scrolling tiled window manager. It provides mostly the same
functionality for MacOS as the PaperWM Gnome extension does for Linux.

Myself and a small group of people have been using this Spoon daily at
https://github.com/mogenson/PaperWM.spoon for awhile. I think it has
reached enough stability to be incorporated into the offical Hammerspoon
Spoons repo.