add extra_config_file

and `--config` alias

for loading a single extra config file, e.g.

    ipython notebook --config mycfg.py

which will be strictly in addition to, and at higher priority than, existing config files, profiles, etc.

Note that the loading code hasn't changed, it's just indented to put it in a for-loop.

IPython/core/application.py

@@ -39,7 +39,7 b' from IPython.config.loader import ConfigFileNotFound'
 from IPython.core import release, crashhandler
 from IPython.core.profiledir import ProfileDir, ProfileDirError
 from IPython.utils.path import get_ipython_dir, get_ipython_package_dir
-from IPython.utils.traitlets import List, Unicode, Type, Bool, Dict
+from IPython.utils.traitlets import List, Unicode, Type, Bool, Dict, Set
 
 #-----------------------------------------------------------------------------
 # Classes and functions
@@ -56,6 +56,7 b' base_aliases = {'
     'profile' : 'BaseIPythonApplication.profile',
     'ipython-dir' : 'BaseIPythonApplication.ipython_dir',
     'log-level' : 'Application.log_level',
+    'config' : 'BaseIPythonApplication.extra_config_file',
 }
 
 base_flags = dict(
@@ -84,14 +85,14 b' class BaseIPythonApplication(Application):'
 
     # Track whether the config_file has changed,
     # because some logic happens only if we aren't using the default.
-    config_file_specified = Bool(False)
+    config_file_specified = Set()
 
     config_file_name = Unicode(u'ipython_config.py')
     def _config_file_name_default(self):
         return self.name.replace('-','_') + u'_config.py'
     def _config_file_name_changed(self, name, old, new):
         if new != old:
-            self.config_file_specified = True
+            self.config_file_specified.add(new)
 
     # The directory that contains IPython's builtin profiles.
     builtin_profile_dir = Unicode(
@@ -102,6 +103,19 b' class BaseIPythonApplication(Application):'
     def _config_file_paths_default(self):
         return [os.getcwdu()]
 
+    extra_config_file = Unicode(config=True,
+    help="""Path to an extra config file to load.
+    
+    If specified, load this config file in addition to any other IPython config.
+    """)
+    def _extra_config_file_changed(self, name, old, new):
+        try:
+            self.config_files.remove(old)
+        except ValueError:
+            pass
+        self.config_file_specified.add(new)
+        self.config_files.append(new)
+
     profile = Unicode(u'default', config=True,
         help="""The IPython profile to use."""
     )
@@ -216,24 +230,25 b' class BaseIPythonApplication(Application):'
             # ignore errors loading parent
             self.log.debug("Config file %s not found", base_config)
             pass
-        if self.config_file_name == base_config:
-            # don't load secondary config
-            return
+        
+        for config_file_name in self.config_files:
+            if not config_file_name or config_file_name == base_config:
+                continue
         self.log.debug("Attempting to load config file: %s" %
                        self.config_file_name)
         try:
             Application.load_config_file(
                 self,
-                self.config_file_name,
+                    config_file_name,
                 path=self.config_file_paths
             )
         except ConfigFileNotFound:
             # Only warn if the default config file was NOT being used.
-            if self.config_file_specified:
+                if config_file_name in self.config_file_specified:
                 msg = self.log.warn
             else:
                 msg = self.log.debug
-            msg("Config file not found, skipping: %s", self.config_file_name)
+                msg("Config file not found, skipping: %s", config_file_name)
         except:
             # For testing purposes.
             if not suppress_errors: