Add clean script

Author: mjackson

package.json

@@ -10,6 +10,7 @@
   "license": "MIT",
   "scripts": {
     "build": "node ./scripts/build.js",
+    "clean": "git clean -fdX .",
     "build-cjs": "babel ./modules -d . --ignore '__tests__'",
     "build-umd": "webpack modules/index.js umd/react-broadcast.js",
     "build-min": "webpack -p modules/index.js umd/react-broadcast.min.js",