Examine source code of Simply Invoice

{
  "name": "Simply Invoice",
  "version": "1.1",
  "manifest_version": 2,
  "browser_action": {
    "default_popup": "index.html",
    "default_icon": {
      "16": "16.png",
      "48": "48.png",
      "128": "128.png"
    }
  },
  "background": {
    "scripts": [
      "background.js"
    ]
  },
  "content_scripts": [
    {
      "matches": [
        "<all_urls>"
      ],
      "js": [
        "content-script.js"
      ]
    }
  ],
  "icons": {
    "16": "16.png",
    "48": "48.png",
    "128": "128.png"
  }
}