Look for podcast description in generic extractor.

Tries sequentially to find common tags containing episode descriptions
for podcasts when using the generic extractor. Silently allows
description to be None if there are not any found.

youtube_dl/extractor/generic.py

@@ -2185,10 +2185,19 @@ class GenericIE(InfoExtractor):
             if not next_url:
                 continue
 
+            description = None
+            for tag in ('description', 'content:encoded', 'itunes:summary', 'itunes:subtitle'):
+                try:
+                    description = it.find(tag).text
+                    break
+                except AttributeError:
+                    pass
+
             entries.append({
                 '_type': 'url_transparent',
                 'url': next_url,
                 'title': it.find('title').text,
+                'description': description
             })
 
         return {