re-enable pydb flag

Note that pydb has been deprecated, and superseded by pydbgr, which may be
abandoned.

It would be preferable if the Pdb class could inherit from pydb or pdb
based on a runtime flag rather than checking sys.argv at the top level.
This at least restores old behavior for pydb users.

closes  #636

IPython/core/debugger.py

@@ -38,7 +38,7 b' from IPython.core.excolors import exception_colors'
 has_pydb = False
 prompt = 'ipdb> '
 #We have to check this directly from sys.argv, config struct not yet available
-if '-pydb' in sys.argv:
+if '--pydb' in sys.argv:
     try:
         import pydb
         if hasattr(pydb.pydb, "runl") and pydb.version>'1.17':

IPython/core/shellapp.py

@@ -50,6 +50,14 b" addflag('pdb', 'InteractiveShell.pdb',"
     "Enable auto calling the pdb debugger after every exception.",
     "Disable auto calling the pdb debugger after every exception."
 )
+# pydb flag doesn't do any config, as core.debugger switches on import,
+# which is before parsing.  This just allows the flag to be passed.
+shell_flags.update(dict(
+    pydb = ({},
+        """"Use the third party 'pydb' package as debugger, instead of pdb.
+        Requires that pydb is installed."""
+    )
+))
 addflag('pprint', 'PlainTextFormatter.pprint',
     "Enable auto pretty printing of results.",
     "Disable auto auto pretty printing of results."