A Piet interpreter written in Go.
Piet is a silly esoteric programming language in which programs are images,
rather than text files.
See http://www.dangermouse.net/esoteric/piet.html for the language spec and sample programs.
Creates a new Piet interpreter for the given image.
The interpreter will use os.Stdin and os.Stdout, but these
can be changed (for example, for testing) by setting the
interpreter's Reader or Writer fields.