[periscope] Add extractor (Closes #5850, closes #6459)

youtube_dl/extractor/__init__.py

@@ -432,6 +432,7 @@ from .orf import (
 from .parliamentliveuk import ParliamentLiveUKIE
 from .patreon import PatreonIE
 from .pbs import PBSIE
+from .periscope import PeriscopeIE
 from .philharmoniedeparis import PhilharmonieDeParisIE
 from .phoenix import PhoenixIE
 from .photobucket import PhotobucketIE