add allowfullscreen to youtube embed

src/Linkification/Linkify.coffee

@@ -397,8 +397,10 @@ Linkify =
     YouTube:
       regExp: /.*(?:youtu.be\/|youtube.*v=|youtube.*\/embed\/|youtube.*\/v\/|youtube.*videos\/)([^#\&\?]*)\??(t\=.*)?/
       el: (a) ->
-        $.el 'iframe',
+        ret = $.el 'iframe',
           src: "//www.youtube.com/embed/#{a.dataset.uid}#{if a.dataset.option then '#' + a.dataset.option else ''}?wmode=opaque"
+        ret.setAttribute "allowfullscreen", "true"
+        ret
       title:
         api: (uid) -> "https://gdata.youtube.com/feeds/api/videos/#{uid}?alt=json&fields=title/text(),yt:noembed,app:control/yt:state/@reasonCode"
         text: (data) -> data.entry.title.$t