Sturdier Any size fan mount for 20mm Extrusion

I needed the fan mounting holes to be a litte more sturdy, so I added two more option to the SCAD:
-Define the outer diameter for the mounting holes -> More plastics, more strength
-Make the bottom of the mounting holes square, to get rid of overhangs there

aaand added it to Customizer for fun :-)

Thanks go to iquizzle for his awesome fan mount system :-)
http://www.thingiverse.com/thing:39945