display default and range in usage

eg. we now see:

$ vips gamma
gamma an image
usage:
   gamma in out
where:
   in           - Input image, input VipsImage
   out          - Output image, output VipsImage
optional arguments:
   exponent     - Gamma factor, input gdouble
  			default: 2.4
			min: 1e-06, max: 1000
operation flags: sequential-unbuffered