Provide a sample tool, contrib/parity-gen, for RAID-like parity sets.

parity-gen will use the par2 command to generate redundant data files, to
be stored with backup segments, that will allow segments to recover even if
some of the data is lost or damaged.  When given a directory, it will
incrementally update parity sets to reflect changes made to the directory
since the last run.

This tool is still under development, and shouldn't be completely trusted
yet.