Examine source code of Random Good Sign-Off

{
  "manifest_version": 2,
  "name": "Random Good Sign-Off",
  "version": "0.1",
  "description": "Click the extension button to insert a good sign-off message randomly chosen from Meg Miller and Hannah Barbera's Are.na channel https://www.are.na/meg-miller/good-sign-offs to your gmail editor",
  "homepage_url": "https://www.are.na/meg-miller/good-sign-offs",
  "icons": {
    "48": "icons/signoff-48.png",
    "96": "icons/signoff-96.png"
  },
  "permissions": [
    "activeTab",
    "*://*.are.na/*"
  ],
  "browser_action": {
    "default_icon": "icons/signoff-48.png",
    "default_title": "Random Good Sign-Off"
  },
  "background": {
    "scripts": [
      "scripts/background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "https://mail.google.com/*",
        "http://mail.google.com/*"
      ],
      "js": [
        "scripts/signoff.js"
      ]
    }
  ]
}