Portable bitmap support

[ynse01]

I'm playing around with implementing support for the Portable Bitmap file format. These include the pgm, pbm, ppm and pam file extensions. See wikipedia.

The format is relatively simple to implement. This fact causes it to be popular in scientific software. Could this be an useful addition to ImageSharp also?

If so, I would like to give it a try and make my code into a Pull Request.

[antonfirsov]

Considering the simplicity of the format, I don't see any disadvantages from adding it to the codebase, for me it looks like a nice addition. @JimBobSquarePants thoughts?

[JimBobSquarePants]

Yeah, fine by me.

[ynse01]

Working on this in PR #1851