appimagetool: option to use Zopfli for better gzip compression

[Zopfli compression](https://en.wikipedia.org/wiki/Zopfli) is basically GZIP on steroids. Zopfli produces compressed files that are [4-8% smaller](https://www.techrepublic.com/blog/the-enterprise-cloud/googles-zopfli-compression-algorithm-extract-higher-performance-from-your-compressed-files/) than those produced by zlib, but which are still valid GZIP files that can be decompressed by existing tools.

The smaller size is achieved at the expense of a __much longer compression time__, meaning that Zopfli is probably not suitable for development or continuous builds that are not used by many people.

However, __decompression remains just as fast__ as with normal GZIP algorithms, so Zopfli compression is ideal for release builds, which are compiled once and then downloaded many thousands of times.

### Compression Level option for `appimagetool`

It would be good to be able to specify the compression level as well as the compression algorithm used by `appimagetool` for creating AppImages.

The compression level option should be algorithm-agnostic. Levels could be `--lowest` (or `--worst` / `--fastest`) ranging to `--highest` (or `--best` / `--slowest`).

Zopfli could either be offered as a separate algorithm to GZIP, or it could be implemented so that choosing the slowest setting with GZIP would actually use Zopfli compression.