Add help option.

When the user enter `?`, the test fails and the user see an "error" message, with the `warning` message if the prompt have it defined.

This is not intuitive, because in the first message say: `Note that answering "?" to any question will show question-specific help`, and it's showing you the error message, not a help message.

My proposal is to add a `help` option (like the `warning` option), and show it if the user answer `?`. If the question don't have `help`, maybe the current behaviour is ok.