Added new plugin to enhance privacy with YouTube video embeds.

This plugin is a conceptual replication of the Electronic Frontier
Foundation's MyTube plugin, which pre-caches thumbnails of embedded
YouTube videos so that users are not tracked by Google until they
explicitly "opt-in" by clicking on the thumbnail (at which point the
thumbnail is replaced by the video embed iframe).
Merge branch 'master' of https://github.com/publicus/pelican-plugins