plugins/unpack-generic.plugin: made it able to unpack xz files.

Actually, took advantage of tar's ability to figure out how a file
is compressed for itself, and merged all of the different kinds of
tar unpack to a single command line.
unpack-generic.plugin: Reinstated zip file support, whoops
Merge pull request #91 from dagbrown/tar_automatic_detection

plugins/unpack-generic.plugin: made it able to unpack xz files.