New: mkvtoolnix-7.9.0-1

[David Stacey]

The following packages have been added to the Cygwin distribution:

* mkvtoolnix-7.9.0-1
* mkvtoolnix-gui-7.9.0-1

MKVToolNix is a set of tools to create, alter and inspect Matroska
files (*.mkv, *.mka).

Dave.