message: Add CSS linting
diff: {
  "scripts": {
  - "test": "xo && cross-env BABEL_ENV=testing ava && run-s build:minified",
  + "lint": "xo && stylelint source/*.css",
  + "test": "npm run lint && cross-env BABEL_ENV=testing ava && run-s build:minified",
  "build": "webpack",
  "build:minified": "cross-env NODE_ENV=production webpack",
  "watch": "webpack --watch",
  "cross-env": "^5.0.5",
  "dot-json": "^1.0.3",
  "npm-run-all": "^4.1.1",
  + "stylelint": "^8.3.1",
  + "stylelint-config-xo": "^0.3.1",
  "uglifyjs-webpack-plugin": "^1.1.2",
  "webext": "^1.9.1-with-submit.1",
  "webpack": "^3.7.1",
  ]
  }
  }
  + },
  + "stylelint": {
  + "extends": "stylelint-config-xo",
  + "rules": {
  + "declaration-no-important": null,
  + "property-no-vendor-prefix": null,
  + "no-descending-specificity": null,
  + "property-blacklist": null,
  + "selector-class-pattern": null,
  + "selector-max-class": null,
  + "rule-empty-line-before": null,
  + "at-rule-empty-line-before": null,
  + "selector-max-compound-selectors": null
  + }
  }
  }
  