make "npm run watch" work on windows (#878)

package.json

@@ -102,6 +102,6 @@
     "test": "standard && depcheck --ignores=babel-cli,nodemon,gh-release --ignore-dirs=build,dist && node ./bin/extra-lint.js",
     "gh-release": "gh-release",
     "update-authors": "./bin/update-authors.sh",
-    "watch": "nodemon --exec 'npm run start' --ext js,pug,css"
+    "watch": "nodemon --exec \"npm run start\" --ext js,pug,css"
   }
 }