DEV: AST transformers supplied to `InteractiveShell` can reject input....
DEV: AST transformers supplied to `InteractiveShell` can reject input.
* Adds a new exception class, `InputRejected` to IPython.lib.security.
* Updates `InteractiveShell.run_cell` and
`InteractiveShell.transform_ast` to allow user-supplied AST
transformers to reject input by raising an `InputRejected`.
* Adds tests for the above behavior.
