Prusa i3 extruder stiffener

This is to stiffen the print head assembly on a Prusa i3 Migbot. It also provides for a strain relief for the wires. Bolts down to the bottom, but will also work ok without the bolt. I also included the Autocad DWG solids file in case you need to modify it some, many extruder stepper motors will not be the same distance from the linear rails.