Add support for PNM

[Roughsketch]

This is still a work in progress. Most of it works, but there is a case in a P6 file where there are no newlines which breaks the parsing since I wrote it assuming that things would always be broken by newlines.

I do have open questions for this for anyone who cares (@akonneker?)

• Should the ImageType be:
  • Pnm
  • Pnm(subtype)
  • Split them into Pbm, Pgm, Ppm
  • Something else?
• Should I handle more than P1 - P6, and if so do you have example files? I've seen P7, PF, PF4, and there are probably more I'm not aware of.

Resolves #14

[Roughsketch]

Going to merge this as-is since it passes all the tests for the files I have now.

I have chosen to keep Pnm as its own value but I am still open to changing it later down the road if needed.