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/embedthumbnail.py

@@ -16,6 +16,7 @@
 except ImportError:
     has_mutagen = False
 
+from .common import PostProcessor
 from .ffmpeg import (
     FFmpegPostProcessor,
     FFmpegThumbnailsConvertorPP,
@@ -62,6 +63,7 @@ def guess():
     def _report_run(self, exe, filename):
         self.to_screen('%s: Adding thumbnail to "%s"' % (exe, filename))
 
+    @PostProcessor._restrict_to(images=False)
     def run(self, info):
         filename = info['filepath']
         temp_filename = prepend_extension(filename, 'temp')