In dealing with sophisticated MS Word files, it turns out that there are many member elements of the pkzip archive that mat2 doesn't currently know what to do with.
This series is an attempt to make the handling of those unknown members a bit more robust, so that it's easier to get reports about missing handlers, and so that people have a clear choice about what to do with them.
This allows the user to make use of parser.unknown_member_policy for
archive formats.
At the suggestion of @jvoisin, it also prints a scary warning if the
user explicitly chooses 'keep'.
