Added files for new Youtube Privacy Enhancer Pelican plugin.

Hello,

I've created a new plugin for Pelican, and would like it to be added to the pelican-plugins repo.

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).

Thank you for your consideration!

Note: I originally made this pull request at https://github.com/getpelican/pelican-plugins/pull/183, but then closed that request after realizing that I should have put the changes in their own branch.