Add option for setting webpack watch option polling

Created by: AJamesPhillips

The motivation was for enabling using a virtualbox virtual machine with hot code reloading. The rsync and normal file synchronisation provided by vagrant 1.9.1 and 5.1.14 does not trigger webpack to hot reload the code. By being able to provide an optional environment variable hot reloading works with vagrant/virtualbox. For example:

$ WEBPACK_WATCH_POLL=1000 npm start

Problem and solution discussed by others here: http://stackoverflow.com/questions/34937294/how-to-enable-hot-reloading-via-shared-folders-with-vagrant-vm#comment64629465_34937378