Add support for Farbfeld images
This patch attempts to add support for the Farbfeld image format. The specification is fairly simple - 16 byte headers with "farbeld" as a magic number and then 4 bytes for width/height each. Everything after that is uncompressed 16-bit RGBA values. An overview of its uses can be found here: https://en.everybodywiki.com/Farbfeld#Implementations
(If it's good enough for ImageMagick, it's good enough for me!)
Sample File: out.ff