add autoplay attribute to custom embed iframe tag, so autoplay works as expected

calzone said i should make a pull request. so here it is.
it would be great if 'allow="autoplay"' attribute is added to the generated iframe for custom embeds.
so autoplay works as expected.

player/embed.coffee

@@ -59,6 +59,7 @@ window.EmbedPlayer = class EmbedPlayer extends Player
             iframe = $('<iframe/>').attr(
                 src: embed.src
                 frameborder: '0'
+                allow: 'autoplay'
                 allowfullscreen: '1'
             )