write JPEG with optimized coding.

According to several sources (https://github.com/jcupitt/libvips/pull/76), enabling this flag allow to produce JPEG with a 3-5 % smaller size. This is what tools like `jpegtran` or `jpegoptim` use.

Being able to specify it could be very interesting for software that want to produce smallest possible JPEGs (i.e. image servers).