Customizable Shaft Buddy: A shaft coupler/adapter generator

**Overview:** This customizer can generate a shaft coupler that accepts a shaft (female) or acts as one (male) on either end for a variety of shaft shapes (hex, square, round (with set screws), and round with one or two flats).

**"Sleeve" mode:** In addition to generating the usual two-sided couplers, it has a special mode to adapt a smaller shaft of one form to a significantly larger one -- it can create a single-form "sleeve" that goes around the smaller shaft, boosting it up to the larger shaft size. To use this mode, set shaft 1 to male, shaft 2 to female, and make sure that shaft 1 is significantly larger than shaft 2. You can disable this special behavior by setting disable_single_form to "yes".

**Background:** A while back, I wrote [a simple set-screw coupler generator for round shafts](http://www.thingiverse.com/thing:1101036). Then I needed to convert a 5/16" round shaft with a 2mm keyway to a 1/2" hex shaft, so I wrote a one-off SCAD script to do that. Then I needed yet