[nerdcubed] Add new extractor

nerdcubed.co.uk describes videos in a single a feed.json file, providing
references to and metadata on >1300 YouTube videos spread across 3 main
channels as well as guest appareances on other channels via a single HTTP
request.

NerdCubedFeedIE transforms this feed into a youtube-dl playlist, preserving
information present in the upstream JSON (allowing zero-cost title/date
matches) and ultimately referencing the embedded YouTube videos.

youtube_dl/extractor/__init__.py

@@ -264,6 +264,7 @@ from .nbc import (
 )
 from .ndr import NDRIE
 from .ndtv import NDTVIE
+from .nerdcubed import NerdCubedFeedIE
 from .newgrounds import NewgroundsIE
 from .newstube import NewstubeIE
 from .nfb import NFBIE