allow empty strings in kv_pattern

doing ipython foo="" would fail as invalid.

IPython/config/loader.py

@@ -307,7 +307,7 b' class CommandLineConfigLoader(ConfigLoader):'
     here.
     """
 
-kv_pattern = re.compile(r'[A-Za-z]\w*(\.\w+)*\=.+')
+kv_pattern = re.compile(r'[A-Za-z]\w*(\.\w+)*\=.*')
 flag_pattern = re.compile(r'\-\-\w+(\-\w)*')
 
 class KeyValueConfigLoader(CommandLineConfigLoader):