When archiving audio or video files, the content is already compressed,
and there is no point in trying to compress it again. Creating a simple
.tar archive is more appropriate in that case.

In mytarc, add an option (-n) so that the archive is not compressed
(create .tar instead of .tar.xz).

In mytarx, check the extension of the archive, and extract accordingly.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>