Allow to specify the number of tasks to run on the command line

You can pass the -t option specifying how many VOBs to encode at a time,
overriding the built in logic.

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

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Update the ffmpeg command

-deinterlace is marked as deprecated, use "-filter:v yadiff" instead
and explicitly specify the video for the video bitrate, i.e "-b:v"

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