new `ipython history clear` subcommand

This new subcommand is equivalent to `ipython history trim --IPythonHistory.keep=0`, which is now supported (and also has a more convenient `--keep` alias)

also included is a tiny fix for nbconvert completion that was pointed out by @jakobgager in #4528

also included is a change to our utils.io.ask_yes_no function that allows for specifying the behavior of a KeyboardInterrupt.