add cover script

Author: shannonmoeller

package.json

@@ -4,7 +4,8 @@
   "description": "Turns off all rules that are unnecessary or might conflict with Prettier.",
   "scripts": {
     "lint": "npx eslint --fix \"**/*.js\" && npx prettier --write \"**/*.js\"",
-    "test": "node test/checker.js"
+    "test": "node test/checker.js",
+    "cover": "npx c8 npm test"
   },
   "keywords": [
     "stylelint",