Start webbrowser in a thread. Prevents lockup with Chrome. If a user has Chrome set as their default browser (system-wide or via the `BROWSER` environment variable), opening the notebook hangs because the chrome call doesn't return immediately. This solves the issue by opening the browser in a thread. Note that there remains an issue where killing the notebook will kill Chrome if the Chrome session was started by us. I haven't found a way to work around that despite attempts by making the webbrowser.open() call in a subprocess.