GIF output should optionally support lossy compression to reduce the file size

[scotchfield]

Slack has an upload limit of 64kb for new emoji images. This means that even a 128x128 image could exceed the file size limit and require a third-party tool to import. It would be nice if party-party-party could ensure the output was optimized and the file size was as small as possible, perhaps by adding a lossy compression parameter.