[rai] Add support for Rai websites (Closes #2930)

youtube_dl/extractor/__init__.py

@@ -216,6 +216,7 @@ from .pornotube import PornotubeIE
 from .prosiebensat1 import ProSiebenSat1IE
 from .pyvideo import PyvideoIE
 from .radiofrance import RadioFranceIE
+from .rai import RaiIE
 from .rbmaradio import RBMARadioIE
 from .redtube import RedTubeIE
 from .ringtv import RingTVIE