Support .xz format

[LaurensRietveld]

A new gzip-like format, used by openphacts
LOD Laundromat does not manage to clean these files (a DirectoryExistenceError is thrown)

After adding it, make sure to add openphacts to the seedlist as well: http://data.openphacts.org/1.5/rdf/

[wouterbeek]

I was under the impression that we were already supporting all libarchive formats. Apparently this one gives a problem in the current implementation. This is probably easy to fix since Jan's API to libarchive has proven to be rather versatile.