Faster/Smaller output with `gifsicle`

I've added an option to provide fast output by using `gifsicle` instead of `convert`. This is orders of magnitude faster than using `convert`, however the quality suffers. Nevertheless, it may be useful when speed is more important than quality.

Additionally, I've piped the default output through `gifsicle`. This reduces the file size marginally. This is an additional dependency, so it might be preferable to allow printing to `STDOUT`, for piping purposes rather than including this optimization directly in the script.