A new feature which writes the video's metadata (title, url, description, etc.)
to the file's extended attributes (xattrs).

A 'utils.write_xattr' function was created which uses whatever method of writing
xattrs exists on the current platform. (First it tries the 'xattr' module,
then the 'setfattr' commandline tool.)

If neither of these facilities are found, the function isn't undefined,
and the metadata writer does nothing.