changes to hidden namespace on startup

Previously, all names declared in code run at startup (startup files, ipython -i script.py, etc.) were added to the hidden namespace, which hides the names from tools like %whos. There are two changes to this behavior:

1. Scripts run on the command-line ipython -i script.py``now behave the same as if they were passed to ``%run, so their variables are never hidden.
2. A boolean config flag InteractiveShellApp.hide_initial_ns has been added to optionally disable the hidden behavior altogether. The default behavior is unchanged.