Improve zip compression
As mentionned in #107, mat2 is currently using the default
ZIP_STORED compression method for all zipfiles.
Maybe we should instead use the same method as the one used by the file being cleaned. This would make fingerprinting a bit easier, but could also dramatically decrease produced archive sizes. I think that it's worth it.
This is a good first issue, since it involves designing a proper integration of this feature in the already-quite-complex archive handling code :)