Polls: add open-choice polls

[andrewlin16]

Allow polls with just a question and no choices, where voters can add their own choices.

If we really want this feature/enhancement, we'll need to make sure to handle the input to /vote commands properly and how to handle cases (e.g. do we do number => vote for existing choice and string => vote for new choice, do we still allow substring matches in that case, should we overload /vote for new choices instead of adding a new command for adding poll choices, ...).