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.
Merge branch 'master' into 1417-local

Conflicts:
	lib/linguist/samples.json
Merge pull request #1459 from github/1417-local

1417 local