#1 adding other quantization methods and dither amount control

[Roy Tam]

I think it is nice to add other quantization methods (e.g. Floyd-Steinberg diffusion dithering and ordered dithering from pngquant, octree, median cut, scolorq) for users to see the difference and choose the best fit method for target image.
And so do the dither amount. Some people may think target image should not be too dithered and be controllable through command line argument.