Pause Button for Shapeoko

[Julien]

@gmack

Maybe @Julien might want to hook it up, test, document, and use it?

• Document it: check (well sort of, I do mention the Feedhold input in anatomy of a Shapeoko section)
• Use it: it’s in my TODO list to finally wire it, and I need to go meddle with my controller wiring soon anyway, so I will (and therefore document it better, in the process)