Allow the IPython command line to run *.ipynb files

[tonyfast]

This PR allows _exec_file in the shell app to pass ipynb files to safe_execfile_ipy in the interactiveshell. safe_execfile_ipy already has the machinery to consume notebooks.

[dsblank]

Does this only work if the notebook has a Python kernel? If this PR isn't accepted, what is the recommended way of running a notebook?

[tonyfast]

This approach will only work for an IPython interactive shell. The run_cell method is invoked so all of the interactive shell rules apply.

The current way of running notebooks is using nbconvert with the --execute flag. The runipy project is deprecated, and recommends the nbconvert approach. Some other ways to acheive a similar result is with importnb or papermill.

[Carreau]

Thanks ! Looks good.