[nerdcubed] Add new extractor

[nerdcubed.co.uk](http://www.nerdcubed.co.uk/) describes videos in a single a [feed.json file](http://www.nerdcubed.co.uk/feed.json), 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.