control psu from the arduino/ramps

@janot928 Just in case you weren't aware, do you know why ATX Power Supplies were used in most of the first designs for OpenSource 3D Printers? It was because that is what the designers had on hand! With the advent of inexpensive higher capacity single rail power supplies very few people use ATX Power Supplies any more. Plus the fact that ATX Power Supplies are rated for the overall wattage across 12v, 5v and 3v rails combined means they rarely have the capacity that most people believe they do.

Now something that might work better, since I have thought about this before would be a SSR or actual relay powered by the PS-ON pins that could switch a single rail power supply on or off. If not better, more useful in general since the 3D printing world has moved past ATX Power Supplies.

Please note that for anything like this to work, your arduino must be powered by an external source 100% of the time.