Add "clean" script

Author: mjackson

package.json

@@ -7,6 +7,7 @@
   "author": "React Training LLC",
   "license": "GPL-3.0",
   "scripts": {
+    "clean": "git clean -fdX .",
     "start": "node ./scripts/build.js && webpack-dev-server --inline --content-base public",
     "ssr-exercise": "supervisor -- -r @babel/register 'subjects/13-Server-Rendering/exercise/server.js'",
     "ssr-solution": "supervisor -- -r @babel/register 'subjects/13-Server-Rendering/solution/server.js'"