Add npm clean script

Author: mjackson

.gitignore

@@ -1 +1,6 @@
+node_modules
 umd
+/*.js
+!karma.conf.js
+!tests.webpack.js
+!webpack.config.js

package.json

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