add "Play videos" button in toolbar

forked from https://github.com/zaius/youtube_playlist/, Plays Youtube videos linked from the page in an overlay HTML5 player, with autoplay. Nice for continuous stream of music, documentaries, talks...

You can see some working examples at:
* https://telecom.dmz.se/links/?searchtags=documentaire
* https://telecom.dmz.se/links/?searchtags=video
* https://telecom.dmz.se/links/?searchtags=music
* https://telecom.dmz.se/links/?searchtags=acoustic

This closes https://github.com/sebsauvage/Shaarli/pull/162

As I said earlier, there is ongoing work on youtube_playlist.js to have it support more websites (vimeo and soundcloud are the first planned)