Extend to support APNG -- animated PNGs

I find myself needing more than 256 colors per frame, so I need the same code to be able to output to either GIF or APNG format.

TODO: Extend `gifwrap` to support the APNG format to be able to read and write animated PNGs as well as GIFs, through the same encoder/decoder interface. The dev should only have to choose the encoder or decoder to select between GIF and APNG.

Hmm. That would leave the package name `gifwrap`a bit deceiving. It might need a more generic name. This might also mean making changes that aren't backward-compatible.