New plugin: media (#676)

The media plugin can autoplay and autopause/autostop <audio> and <video> elements when entering and leaving a step.

Support for impressConsole: don't autoplay in preview window and play but mute clips in current window.

src/plugins/impressConsole/impressConsole.js

@@ -555,7 +555,7 @@
             //Btw, you can also launch console automatically:
             //<div id="impress" data-console-autolaunch="true">
             if ( root.dataset.consoleAutolaunch === 'true' ) {
-                window.open();
+                open();
             }
         };