Support compressed ISOs

Hi,

when adding Guix to iosinfo-db (!202 (merged)), I couldn't add the path to the ISO file, because Guix only serves a compressed ISO (guix-system-install-1.1.0.i686-linux.iso.xz). It would be interesting to have a mechanism to mark a URL as being for a compressed ISO, and add a way to uncompress it to get the ISO itself.

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