polls: Add an option to a poll

[gnprice]

As part of poll support (#164), we should support adding an option to a poll.

This is something that anyone seeing the poll message can do, not only the poll creator; see https://zulip.com/help/create-a-poll .

This will come after #166. It corresponds to an open feature request in zulip-mobile:

• Allow adding new poll options zulip-mobile#4883