Feature: support independent outputs

[ppenguin]

I wanted to settle for enkei (and add an option to use it for styli.sh) since it focuses on functionality that makes sense for my use case: applying a wallpaper from a filename argument to an output (and offering a nice transition when a new image is applied by a one-shot control process (in this case enkeictl).

Pretty much the (only?) thing missing to satisfy the use case is ability to select outputs.
(I took a look at it, but I'm not up to the task myself, never having done any rust before).

(FWIW: I feel (too?) that alternatives focus on the "wrong" things by e.g. needing static configs (hyprpaper, wpaperd), or not offering "stand-alone" usage with good rendering but avoiding "choosing" functionality best handled by external programs, for which I want to use styli.sh. swaybg would come closest but no smooth transitions and for me output selection (on hyprland) didn't work.)