Add contours for more "comic look"

[h4gen]

I played around a bit with active black contours like here before and after the application of the transformation and I think it would be a great addition to the library and should be pretty straight forward to implement. It can give a more comic-like look to the results.

[sedthh]

@Seon82 had this idea of running edge detection on the images before applying the algorithm, so I was thinking about experimenting with canny.

But this method can also work (most likely afterwards on dark pixels). Thanks for the tip, I will look into it once I'll have some free time.