Optionally filter all images for a different style

[pserwylo]

There are many good tools for image processing, to produce cartoon style graphics from a source image (e.g. https://towardsdatascience.com/using-opencv-to-catoonize-an-image-1211473941b6). By doing so, we not only have the option of making our books adopt unique styles (e.g. hand drawn, colourful, etc), but also unify the style of all images. Right now images are all from different authors, cameras, styles and the resulting books can reflect this randomness.