Adds a build:watch npm script.

I was working on cache-loader as an npm linked dependency, I generally use this
script so that I don't have to think about rebuilding between changes.

Author: gdborton

package.json

@@ -11,6 +11,7 @@
   "scripts": {
     "start": "npm run build -- -w",
     "build": "cross-env NODE_ENV=production babel src -d dist --ignore 'src/**/*.test.js' --copy-files",
+    "build:watch": "npm run build -- --watch",
     "clean": "del-cli dist",
     "lint": "eslint --cache src test",
     "lint-staged": "lint-staged",