yagp: Add a -h option for getopt()

By convention the -h option should display the help/usage summary.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
README: Update to cover the -p and -r options

-p is the number of images per page (defaults to 16) and -r is the
number images per row (defaults to 4).

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>