Customizable Menger sponge

Continuing the development of the [Menger sponge] models, and [encouraged] by [mathgrrl], this OpenSCAD script combines the [cut] and [external support] options and adds the ability to change the size and choose the order/level up to 3.

[Menger sponge]: http://en.wikipedia.org/wiki/Menger_sponge
[encouraged]: http://makerhome.blogspot.com
[mathgrrl]: http://www.thingiverse.com/mathgrrl/overview/
[cut]: http://www.thingiverse.com/thing:234989
[external support]: http://www.thingiverse.com/thing:238551

Feb 6 update - added two more capabilities to the script:

* a combination of cut and supported, to print a half-cube with the cut side facing up for folks with difficulty removing the finished object from their printbeds,
* an option for an order zero cube - just a plain cube, no holes - so you can make a set showing the progression from cube to fractal.