Image Slicer

I create an experimental `contours` based on [marching squares](https://en.wikipedia.org/wiki/Marching_squares). Here's a demonstration. You can find source code on [dotSCAD/examples/image_slicer.scad](https://github.com/JustinSDK/dotSCAD/blob/master/examples/image_slicer.scad), too. The source code requires [OpenSCAD](http://openscad.org) and [dotSCAD](https://github.com/JustinSDK/dotSCAD).

If you want to slice your selected picture, use [img2gray](https://github.com/JustinSDK/img2gray) to convert an image into gray scale data. Then, use the gray scale data to replace the `level` value.