ppmgrayscreen: Netpbm filter to establish page monochromaticity

ppmgrayscreen is a filter I wrote long ago to remove extraneous grading marks from a student test I had accidentally marked up. Its basic mode of operation is to find pixels that are not a shade of some target color (default black) and change them to white. It's not great, but better than nothing for this job.

See the source repo for details.