Add support for notebook server extensions

As some people may be tiring of my pointing out, it seems strange that
we have extension points for the JS and the kernel, but none for the
notebook server.

For cite2c, I want to add a handler which can serve a directory of style
files for use by the nbextension part.

IPython/html/notebookapp.py

@@ -9,6 +9,7 @@ from __future__ import print_function
 import base64
 import datetime
 import errno
+import importlib
 import io
 import json
 import logging
@@ -709,6 +710,10 @@ class NotebookApp(BaseIPythonApplication):
         self.config.FileContentsManager.root_dir = new
         self.config.MappingKernelManager.root_dir = new
 
+    extensions = List(Unicode(), config=True,
+        help="Python modules to load as notebook server extensions"
+    )
+
     def parse_command_line(self, argv=None):
         super(NotebookApp, self).parse_command_line(argv)
         
@@ -915,6 +920,22 @@ class NotebookApp(BaseIPythonApplication):
         elif status == 'unclean':
             self.log.warn("components submodule unclean, you may see 404s on static/components")
             self.log.warn("run `setup.py submodule` or `git submodule update` to update")
+
+    def init_extensions(self):
+        """Load any extensions specified by config.
+
+        Import the module, then call the load_jupyter_server_extension function,
+        if one exists.
+        """
+        for modulename in self.extensions:
+            try:
+                mod = importlib.import_module(modulename)
+                func = getattr(mod, 'load_jupyter_server_extension', None)
+                if func is not None:
+                    func(self)
+            except Exception:
+                self.log.warn("Error loading server extension %s", modulename,
+                              exc_info=True)
     
     @catch_config_error
     def initialize(self, argv=None):
@@ -926,6 +947,7 @@ class NotebookApp(BaseIPythonApplication):
         self.init_webapp()
         self.init_terminals()
         self.init_signal()
+        self.init_extensions()
 
     def cleanup_kernels(self):
         """Shutdown all kernels.