[generic] add support for relative URLs (Fixes #1308)

youtube_dl/extractor/generic.py

@@ -165,6 +165,8 @@ class GenericIE(InfoExtractor):
         video_url = compat_urllib_parse.unquote(mobj.group(1))
         if video_url.startswith('//'):
             video_url = compat_urllib_parse_urlparse(url).scheme + ':' + video_url
+        if '://' not in video_url:
+            video_url = url + ('' if url.endswith('/') else '/') + video_url
         video_id = os.path.basename(video_url)
 
         # here's a fun little line of code for you: