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` ;).
def do_skip(self, arg):
"Skip the current question, and continue with the quiz"
+ self.ui.stack.append(self.question)
self.question = self.ui.get_question()
if not self.question:
return True # out of questions