Extend to support APNG -- animated PNGs

[jtlapp]

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 gifwrapa bit deceiving. It might need a more generic name. This might also mean making changes that aren't backward-compatible.

[jtlapp]

I plan to address this by creating a new package that generalizes this one, because it's a breaking change that requires a new package name.