Examine source code of Certage

{
  "manifest_version": 2,
  "name": "Certage",
  "version": "1.2.0",
  "description": "Show the remaining days/hours for the SSL certificate for any web page.",
  "browser_action": {
    "default_icon": {
      "48": "icons/48_green.png"
    },
    "default_title": "Certage",
    "default_popup": "popup/index.html"
  },
  "icons": {
    "48": "icons/48_green.png"
  },
  "permissions": [
    "<all_urls>",
    "storage",
    "webRequest",
    "webRequestBlocking"
  ],
  "background": {
    "scripts": [
      "background.js"
    ]
  }
}