Adds the ability to play youtube videos.

It still needs a bit of work, but playing and pausing are operational.

add-on/manifest.json

@@ -22,6 +22,13 @@
     "scripts": ["background.js"]
   },
 
+  "content_scripts": [
+    {
+      "matches": ["*://*.youtube.com/watch?v=*"],
+      "js": ["foreground.js", "youtube.js"]
+    }
+  ],
+
   "browser_action": {
     "default_icon": "icon.svg"
   },