Add command to show compression statistics (#440)

Add command to show compression statistics (#440)

This will only show information about the files that is knows about
(right magic bytes). So the file count might differ from what is shown
with the regular statistics (which shows all files, including old ones).

The terminology used here is a bit confused, the compression ratio is
supposed to grow upwards. Sometimes known as "space savings" instead,
so list both values (ratio and savings) to make the output more obvious.