[execafterdownload] Simplify (#3569)

youtube_dl/YoutubeDL.py

@@ -172,6 +172,7 @@ class YoutubeDL(object):
     The following options are used by the post processors:
     prefer_ffmpeg:     If True, use ffmpeg instead of avconv if both are available,
                        otherwise prefer avconv.
+    exec_cmd:          Arbitrary command to run after downloading
     """
 
     params = None