Examine source code of Trello progress

{
  "manifest_version": 2,
  "name": "Trello progress",
  "version": "1.1",
  "description": "Adding progressbar to the card's background based on completet tasks count",
  "icons": {
    "48": "icons/trello-progress-48.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://trello.com/*"
      ],
      "js": [
        "trello-progress.js"
      ]
    }
  ]
}