add support for displaying image from url

[shadow-absorber]

add support to show images from urls like https://http.cat/100.jpg/ or similar doing this would mean we could work towards adding ueberzug support into w3m or use ueberzug as an external image viewer

[jstkdng]

interesting feature, you can start with a w3m implementation by having it download the pictures and using u++ to display them in the meantime