Add option to use the original image as mask

[lucasoethe]

This allows the user to create images where the Arch Logo is "imprinted" in a darkened/blurred/translated version.
As an example, going from this image and using
arch_papers -o -b 5 -d 40 -t 0,100 --bg-scale 0.75 input.jpg output.jpg
Yields the following result:

[connorslade]

Thanks for the contribution :p

I will update the docs and make a new release in the next few days.