[ytdl] add extractor for sites supported by youtube-dl

(#1680, #878)

Can be used by prefixing any URL with 'ytdl:',
or by setting 'extractor,ytdl.enabled' to 'true'.

test/test_extractor.py

@@ -147,7 +147,7 @@ class TestExtractorModule(unittest.TestCase):
             return c.capitalize()
 
         for extr in extractor.extractors():
-            if extr.category not in ("", "oauth"):
+            if extr.category not in ("", "oauth", "ytdl"):
                 expected = "{}{}Extractor".format(
                     capitalize(extr.category),
                     capitalize(extr.subcategory),