Examine source code of TwitterAltText

{
  "manifest_version": 2,
  "name": "TwitterAltText",
  "version": "0.3",
  "developer": {
    "name": "Davyd McColl",
    "url": "https://github.com/fluffynuts"
  },
  "description": "Surfaces alt-text for images on Twitter",
  "icons": {
    "48": "icon-48.png",
    "96": "icon-96.png"
  },
  "content_scripts": [
    {
      "matches": [
        "https://twitter.com/*",
        "https://mobile.twitter.com/*"
      ],
      "js": [
        "main.js"
      ]
    }
  ]
}