add a option for "clearing the screen" to ipythonrc

[Timmie]

According to:
http://thread.gmane.org/gmane.comp.python.ipython.user/4701

CTRL-L could be bound to clear-screen

by adding
readline_parse_and_bind "\C-l": clear-screen

to ~/.ipython/ipythonrc

This should be enabled on all platforms.

Thanks.

[Fernando Perez (fdo.perez)]

This can be done as a user customization, and we can not enable it indiscriminately, because on windows and osx the various readline libraries don't necessarily know how to clear the screen.

[Fernando Perez (fdo.perez)]

Actually I take that back, we do have already other complex parse_and_bind calls and we protect against using them in win/osx.

So I just added this in trunk, thanks!