Added Fillet and moved extrusion params to new file

All of the motor and rod holders are now using a fillet for faster
printing and slightly less material. The extrusion parameters have now
been moved to conf_extrusion.scad Look there for the different types
that are pre-defined, and select your extrusion type in
configuration.scad
Please note that when you change your extrusion type, you will want to
change the ex_screw variables too.