Add command-line interface to enable/disable nbextensions

i.e. to enable cite2c, you'd use
`jupyter nbextension enable cite2c/main`.

Open questions:

1. Should these commands accept multiple extension names in one call?
nbextension install currently doesn't, so there's a symmetry argument,
but it would be easy to handle more than one if necessary.

2. Especially for disable, should it give an error if you try to disable
something that's not enabled? Currently it will just pass silently.
Enable could also fail if it's already enabled, but that seems less
useful.

notebook/nbextensions.py

@@ -24,6 +24,7 @@ from jupyter_core.paths import jupyter_data_dir, jupyter_path, SYSTEM_JUPYTER_PA
 from ipython_genutils.path import ensure_dir_exists
 from ipython_genutils.py3compat import string_types, cast_unicode_py2
 from ipython_genutils.tempdir import TemporaryDirectory
+from notebook.services.config import ConfigManager
 from ._version import __version__
 
 class ArgumentConflict(ValueError):
@@ -316,6 +317,57 @@ class InstallNBExtensionApp(JupyterApp):
                 print(str(e), file=sys.stderr)
                 self.exit(1)
 
+
+class EnableNBExtensionApp(JupyterApp):
+    name = "jupyter nbextension enable"
+    version = __version__
+    description = "Configure an nbextension to be automatically loaded"
+
+    section = Unicode('notebook', config=True,
+          help=("Which config section to add the extension to. "
+                "'common' will affect all pages.")
+    )
+
+    aliases = {'section': 'EnableNBExtensionApp.section',
+              }
+
+    def enable_nbextension(self, name):
+        cm = ConfigManager(parent=self, config=self.config)
+        cm.update(self.section, {"load_extensions": {name: True}})
+
+    def start(self):
+        if not self.extra_args:
+            self.print_help()
+            sys.exit('No extensions specified')
+
+        self.enable_nbextension(self.extra_args[0])
+
+
+class DisableNBExtensionApp(JupyterApp):
+    name = "jupyter nbextension disable"
+    version = __version__
+    description = "Remove the configuration to automatically load an extension"
+
+    section = Unicode('notebook', config=True,
+          help=("Which config section to remove the extension from. "
+                "This should match the one it was previously added to.")
+    )
+
+    aliases = {'section': 'DisableNBExtensionApp.section',
+              }
+
+    def disable_nbextension(self, name):
+        cm = ConfigManager(parent=self, config=self.config)
+        # We're using a dict as a set - updating with None removes the key
+        cm.update(self.section, {"load_extensions": {name: None}})
+
+    def start(self):
+        if not self.extra_args:
+            self.print_help()
+            sys.exit('No extensions specified')
+
+        self.disable_nbextension(self.extra_args[0])
+
 class NBExtensionApp(JupyterApp):
     name = "jupyter nbextension"
     version = __version__
@@ -325,6 +377,8 @@ class NBExtensionApp(JupyterApp):
         install=(InstallNBExtensionApp,
             """Install notebook extensions"""
         ),
+        enable=(EnableNBExtensionApp, "Enable a notebook extension"),
+        disable=(DisableNBExtensionApp, "Disable a notebook extension"),
     )
 
     def start(self):