ui.cli: Add a `skip` command to the quizzer shell

This lets you bypass a difficult question on the stack and continue
with the rest of the quiz.
ui.cli: `skip` shifts the current question to the back of the stack

Rather than dropping skipped questions, just save them for later.
This gives you another stab at them without having to restart the
quiz.  If you still can't answer the questions once you reach them
again, you can always `quit` ;).