[hrfernsehen] add extractor

Add an extractor for videos provided by HR Fernsehen, a regional TV
channel in Germany.

Signed-off-by: David Bauer <mail@david-bauer.net>

youtube_dl/extractor/extractors.py

@@ -440,6 +440,7 @@ from .hotstar import (
 )
 from .howcast import HowcastIE
 from .howstuffworks import HowStuffWorksIE
+from .hrfensehen import HRFernsehenIE
 from .hrti import (
     HRTiIE,
     HRTiPlaylistIE,