Allow images formats

Necessary for #343.

* They are identified by `vcodec=acodec='none'`
* These formats show as the worst in `-F`
* Any postprocessor that expects audio/video will be skipped
* `b*` and all related selectors will skip such formats
* This commit also does not add any selector for downloading such formats. They have to be explicitly requested by the `format_id`. Implementation of a selector is left for when #389 is resolved

yt_dlp/postprocessor/sponskrub.py

@@ -41,6 +41,7 @@ class SponSkrubPP(PostProcessor):
                 return None
         return path
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, information):
         if self.path is None:
             return [], information