add `ConsoleWidget.check_complete` flag

disables input-parsing in the QtConsole,
which is inappropriate for non-Python kernels.

Disabling check_complete forces shift-enter to execute, just like the Notebook.
execute_on_complete_input

better name for check_complete
Merge pull request #3814 from minrk/no-complete

add `ConsoleWidget.execute_on_complete_input` flag

disables input-parsing in the QtConsole, which is inappropriate for non-Python kernels.

Disabling execute_on_complete_input requires shift-enter to execute, just like the Notebook.

Not sure if we want this for 1.0 or not. It's pretty trivial, but it is technically a new feature.