re-add `npm run build:watch`

it's not true watching, just polling every few seconds.

Help docs still point to this command,
and we need something for devs as `ignore_minified_js` is gone.

package.json

@@ -10,7 +10,8 @@
   },
   "scripts": {
     "bower": "bower install",
-    "build": "python setup.py js css"
+    "build": "python setup.py js css",
+    "build:watch": "set -e; while true; do npm run build; sleep 3; done"
   },
   "devDependencies": {
     "bower": "*",