Transform to multiple groups support

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Currently, all files generated from a transform are made into one group.

But it is sometimes possible that a single archive file may contain many groups of files that either look similar to each other or shares a common name prefix.

The specific example given was unpacking cg bundles of art rips downloaded from torrent sites.

I never understood why people would try compress already compressed images. But I suppose we could always support finer granularity grouping and ship a separate utility to do phash grouping for small sets of files that can then emit meta-data in an understandable format.