[JWPlatform] Support iframes

Support content.jwplatform... src attributes inside <iframe> tags in
addition to <script> tags. Just a regexp change.

Add a test (currently Generic_72).

youtube_dl/extractor/jwplatform.py

@@ -24,7 +24,7 @@ class JWPlatformIE(InfoExtractor):
     @staticmethod
     def _extract_url(webpage):
         mobj = re.search(
-            r'<script[^>]+?src=["\'](?P<url>(?:https?:)?//content.jwplatform.com/players/[a-zA-Z0-9]{8})',
+            r'<(script|iframe)[^>]+?src=["\'](?P<url>(?:https?:)?//content.jwplatform.com/players/[a-zA-Z0-9]{8})',
             webpage)
         if mobj:
             return mobj.group('url')