Allow passing in traitlets via commandline

Without this, you *always* needed a repo2docker_config.py
file to configure anything. This PR makes r2d match the
behavior of most traitlets based applications (like jupyter_server,
jupyterhub, nbconvert, etc)

Fixes jupyterhub#1112

repo2docker/__main__.py

@@ -251,9 +251,10 @@ def make_r2d(argv=None):
         print(__version__)
         sys.exit(0)
 
-    args = get_argparser().parse_args(argv)
+    args, traitlet_args = get_argparser().parse_known_args(argv)
 
     r2d = Repo2Docker()
+    r2d.parse_command_line(traitlet_args)
 
     if args.debug:
         r2d.log_level = logging.DEBUG

repo2docker/app.py

@@ -490,7 +490,7 @@ def json_excepthook(self, etype, evalue, traceback):
             extra=dict(phase="failed"),
         )
 
-    def initialize(self):
+    def initialize(self, *args, **kwargs):
         """Init repo2docker configuration before start"""
         # FIXME: Remove this function, move it to setters / traitlet reactors
         if self.json_logs: