Store frontend config in nbconfig/ subdirectory

Closes gh-211

notebook/services/config/manager.py

@@ -3,6 +3,8 @@
 # Copyright (c) Jupyter Development Team.
 # Distributed under the terms of the Modified BSD License.
 
+import os.path
+
 from traitlets.config.manager import BaseJSONConfigManager
 from jupyter_core.paths import jupyter_config_dir
 from traitlets import Unicode
@@ -12,4 +14,4 @@ class ConfigManager(BaseJSONConfigManager):
     
     config_dir = Unicode(config=True)
     def _config_dir_default(self):
-        return jupyter_config_dir()
+        return os.path.join(jupyter_config_dir(), 'nbconfig')