- add `negate-input` / `negate-output` toggles from otsu
      to other algorithm CLIs too
      (but keep `negate` as synonym for `negate-output` in sauvola;
       and don't add `negate-input` where this would require
       negation on RGB)
    - use the versatile io::magick::save instead of just io::pbm::save
    - rename output argument to `output.*` to reflect versatility