Add minimal support for recognizing OpenSCAD files.

As 3D printing becomes more popular, more OpenSCAD projects will appear
on github. This change allows linguist to recognize those projects.
Hopefully, this will make finding projects easier.

To show a few examples:
 * https://github.com/gwadej/star_fury_model
 * https://github.com/gwadej/tslot_nut_holder-stl
 * https://github.com/secondsight/secondsight-hardware

For some projects that I'm not associated with:
 * https://github.com/tbuser/openscad-bitmap
 * https://github.com/kellyegan/OpenSCAD-Arduino-Mounting-Library
 * https://github.com/algspd/openscad
 * https://github.com/radavis/openscad
 * https://github.com/bert/openscad-models
 * https://github.com/XarpeSerpe/openscad
 * https://github.com/jleto/openscad
 * https://github.com/fukanchik/openscad