[OpenSCAD] open .stl file by default

[Laszlo KREKACS]

Hi,

This is more a usability fix then a feature request.

openscad ./something.stl should open openscad with the following template:
import_stl("./something.stl");

So I could quickly view .stl files with openscad. There are few other
things missing
to be a really usable .stl viewer  (eg. measuring tools),
but for quick viewing this one is the most urgent "bug".

(also doubleclicking on a .stl file should behave the same if the
system associated openscad with the .stl filetype)

What do you guys thinks?

Best,
 Laszlo