Examine source code of Measure-it

{
  "manifest_version": 2,
  "name": "Measure-it",
  "description": "Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.",
  "version": "2.2.0",
  "applications": {
    "gecko": {
      "id": "{79b2e4de-8fb4-4ccc-b9f6-362ac2fb74b2}"
    }
  },
  "icons": {
    "32": "icons/icon_32.png",
    "48": "icons/icon_48.png",
    "64": "icons/icon_64.png",
    "128": "icons/icon_128.png"
  },
  "browser_action": {
    "default_icon": "icons/icon_32.png",
    "default_title": "Measure-it"
  },
  "background": {
    "scripts": [
      "polyfill.js",
      "bg.js"
    ]
  },
  "options_ui": {
    "page": "options.html",
    "browser_style": true
  },
  "permissions": [
    "activeTab",
    "storage"
  ],
  "commands": {
    "_execute_browser_action": {
      "suggested_key": {
        "default": "Alt+Shift+M",
        "mac": "Command+Shift+O"
      }
    }
  }
}