Adjust tile color balance

[valdanylchuk]

1. Added the ADJUST_COLORS option to adjust each tile's color balance to match the target fragment better. Produces better visual quality with fewer tiles, especially when there are few source tile images.
2. Added a palette of plain color PNG images in the images/colors directory for quick experiments, and for a cool pixel art effect. Also added the script extras/gen_colors_rgb.py that generates those.
3. Added optional reporting of three statistical metrics for the similarity of the target image and the mosaic: MSE, structural similarity index, perceptual hash difference.

Feel free to pick only those parts that you like.