pixz: new package

Add a package for pixz (pronounced pixie), a parallel, indexing version of
xz.

See https://github.com/vasi/pixz



Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>