added --autoreload flag

When passed, the webapp will watch for any changes to its Python source. On
change, all changed packages will be reimported and the webapp will restart.
Also works on Python source files in Jupyter server extensions. Implemented
using the built in `autoreload` parameter in the constructor of
`tornado.web.Application`.

notebook/notebookapp.py

@@ -160,6 +160,9 @@ def __init__(self, jupyter_app, kernel_manager, contents_manager,
             default_url, settings_overrides, jinja_env_options)
         handlers = self.init_handlers(settings)
 
+        if settings['autoreload']:
+            log.info('Autoreload enabled: the webapp will restart when any Python src file changes.')
+
         super(NotebookWebApplication, self).__init__(handlers, **settings)
 
     def init_settings(self, jupyter_app, kernel_manager, contents_manager,
@@ -542,6 +545,16 @@ def start(self):
     _("Allow the notebook to be run from root user.")
 )
 
+flags['autoreload'] = (
+    {'NotebookApp': {'autoreload': True}},
+    """Autoreload the webapp
+    
+    Enable reloading of the tornado webapp and all imported Python packages
+    when any changes are made to any Python src files in Notebook or 
+    extensions.
+    """
+)
+
 # Add notebook manager flags
 flags.update(boolean_flag('script', 'FileContentsManager.save_script',
                'DEPRECATED, IGNORED',
@@ -648,6 +661,10 @@ def _default_log_format(self):
         help=_("Whether to allow the user to run the notebook as root.")
     )
 
+    autoreload = Bool(False, config=True,
+        help= ("Reload the webapp when changes are made to any Python src files.")
+    )
+
     default_url = Unicode('/tree', config=True,
         help=_("The default URL to redirect to from `/`")
     )
@@ -1386,6 +1403,7 @@ def init_webapp(self):
         if self.allow_origin_pat:
             self.tornado_settings['allow_origin_pat'] = re.compile(self.allow_origin_pat)
         self.tornado_settings['allow_credentials'] = self.allow_credentials
+        self.tornado_settings['autoreload'] = self.autoreload
         self.tornado_settings['cookie_options'] = self.cookie_options
         self.tornado_settings['get_secure_cookie_kwargs'] = self.get_secure_cookie_kwargs
         self.tornado_settings['token'] = self.token