[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.