support for PBM/PGM/PPM

[akonneker]

I recently found myself with a dataset containing PPM files, which seems like an especially simple addition to this library based on the format overview on wikipedia.

[Roughsketch]

It actually seems a bit more complex than other formats since I'd need to handle comments and newline/no newline versions. However it doesn't seem that bad. I don't know when I'll have free time to do it, but will add it to the list of formats at some point most likely.

Thanks for the suggestion.