Allow dithering of more bitmap types.

[rsbohn]

The current bitmaptools.dither() requires:

Source bitmap that contains the graphical region to be dithered. It must have a value_count of 65536.

adafruit_imageload complains if you try to load an image with too many colors.

How can I load a truecolor image and dither it for display on epaper?

[tannewt]

ColorConverter already allow you to set the dither attribute to apply a dither over the colors from the source bitmap. I'm adding the ability to Palette as well.