Add support for lossygif

[davidschreiber]

There's a forked version of gifsicle called lossygif (https://kornel.ski/lossygif) that allows for reducing GIF file sizes even more than using the original gifsicle optimizations.

gifsicle --lossy=20 -O3 -o out.gif in.gif

Robogif could use lossygif to even further crunch down sizes of generated GIFs. A very convenient feature would be a --max-filesize option, that would tell robogif to continuously increase lossyness, trying get the final file size below that option value. This option can come in handy when a file size restriction is given (e.g. when adding GIF files to GitHub issues).

robogif --max-filesize=10M out.gif