[utils] Add more codecs to codec2ext

BBC uses avc3. Here's an example (thanks to @remitamine for this example)

http://rdmedia.bbc.co.uk/dash/ondemand/bbb/2/client_manifest-common_init.mpd

See also https://trac.ffmpeg.org/ticket/5217

youtube_dl/utils.py

@@ -1900,6 +1900,9 @@ def codec2ext(codec):
     # is not a good fallback for file extensions
     return {
         'avc1': 'mp4',
+        'avc2': 'mp4',
+        'avc3': 'mp4',
+        'avc4': 'mp4',
         'mp4a': 'm4a',
         'vorbis': 'webm',
         'vp9': 'webm',