Customizable cap nut

**NOTE: This has been superseded by [Customizable - every nut and washer](http://www.thingiverse.com/thing:826065).**

I like kumy's Parametric Nut Cap, but I needed one to thread directly onto a rod as opposed to capturing a normal hex nut, so I modified his script to add this option. I also added a dropdown to select the nut size from standard SAE and metric sizes, and slightly rounded the outer edges for easier wrenching.