Specify content rectangle using margins

[galfar]

Right now you use "-r 100,200,600,700" to explicitly specify the content rectangle in pixels
but useful can be also to specify it using margins e.g. take 20% left and 10% top off - especially when
batch processing similar class of images but differing in sizes pixels.

Best would still be to select the rect visually :)

[galfar]

Also useful would be to specify it physical units i.e. cm/mm and inches since it's closer to the initial source -> the paper. Question is, what to do when physical resolution is not present in the input image. Probably report it and fail to be on the safe side.