Exponential horn generator

This is an exponential horn library for OpenSCAD. I made it to amplify the sound from my son's watch alarm (fittings for that are [here](https://www.thingiverse.com/thing:2500204)). You can use the library in your projects, or just generate horns with the demo Customizer here.

To generate a horn programmatically, do `use ;` and call `horn()`. You will also need my [tubemesh](https://www.thingiverse.com/thing:2406760) library (it's bundled in the demo).

There are lots of optional arguments:

* `length=x`: length of horn, from mouth to throat
* `throat=[w,h]`: width and height of throat (narrow end)
* `mouth=[w,h]`: width and height of mouth (wide end)
* `wallThickness=x`: thickness of walls
* `numSections=n`: number of strips to use (more is slower but better quality)
* `rectangular=b`: set to `false` to do a round/elliptical horn (very slow); default: `true`
* `flangeLength=x`: length of flange around mouth, for mounting mouth into something; set to 0 to omit flang