Set up shell command-line tab-completion for ipython

Set up shell command-line tab-completion using
argcomplete and ipython/traitlets#811

argcomplete supports following setuptools console_scripts
to the corresponding package's __main__.py to look for a
PYTHON_ARGCOMPLETE_OK marker.

IPython/__init__.py

@@ -1,3 +1,4 b''
+# PYTHON_ARGCOMPLETE_OK
 """
 IPython: tools for interactive and parallel computing in Python.
 

IPython/__main__.py

@@ -1,3 +1,4 b''
+# PYTHON_ARGCOMPLETE_OK
 # encoding: utf-8
 """Terminal-based IPython entry point.
 """