Examine source code of StoppingCues Alpha

{
  "manifest_version": 2,
  "name": "StoppingCues Beta",
  "version": "1.0",
  "description": "Adds a stopwatch to twitter.com",
  "content_scripts": [
    {
      "matches": [
        "*://*.twitter.com/*",
        "*://*.facebook.com/*",
        "*://*.instagram.com/*"
      ],
      "js": [
        "timer.js",
        "randomized_comments.js"
      ],
      "css": [
        "timer-styling.css"
      ]
    }
  ]
}