question: Add support for list-of-lines prompts

question: Add support for list-of-lines prompts

We use lists of lines for other question attributes (e.g. setup,
teardown, answer).  Be consistent with multi-line prompts.

The handling is in Question.format_prompt() for easy access by any
consumer.  The `newline` option will make it easy to support
non-terminal output formats (e.g. `newline='<br/>'` for HTML).