Package.json - Added browser config for fs

So as to avoid webpack warnings being thrown when compiling an app with messageformat in, we need to specify a browser definition for fs.

From the creator of weback to jade:
pugjs/pug#1644

package.json

@@ -51,5 +51,8 @@
   },
   "scripts": {
     "test": "make test"
+  },
+  "browser": {
+    "fs": false
   }
 }