Customizable Profile Screw Thread

OpenSCAD module to generate screw threads with any profile. The .STL is an example male/female pair with a trapezium profile.

Arguments are:

* profile: A list of points describing the profile of the thread, like you would pass to polygon()
* pitch
* diameter
* height

The module respects $fa as the minimum angle of a fragment, so optionally pass this to control resolution.

The profile is a list of [x,y] points describing a polygon. x values of 0 will lie on the surface of a cylinder of the appropriate diameter. x > 0 will be recessed, x < 0 will protrude.

**Update 2016-02-13**

The previous version aligned the profile in the plane perpendicular to the thread, which lead to a slightly distorted profile in vertical cross section (increasing proportional to pitch). The latest version aligns the profile in vertical cross section.