Add test case requiring subtracting "arm".

This contains three images: background, an image with an uncompressed
slinky, and an image with a compressed slinky.

Note that fg2.jpg contains two black objects to represent "arms" that
compress the object. Thus the object detector will need to ignore those
areas to correctly identify the resulting compressed object.
Add ignore color method for object detection.

This method marks all colors that fall between two user-specified color
values (converted to HSV space) to be treated as background by other
methods, like the pixel dimension finder. The foreground mask itself
remains unchanged.

This method was applied in the latest test case (example4). A relatively
large color range was used; the ignored areas can be found in
fg2__ignore.png (the white areas).