End Vote After Specified Time

Allow a vote to end after a specified time limit.

Example:
`!vote new Red Blue` Create a vote on whether Red or Blue is better.
`!vote end 2m` Sets a 2 minute timer to end the vote (will override a previous vote ending timer).
`!vote end` Ends a vote immediately.

- Even if time limit already set, vote can still be manually ended early by using the `!vote end` command again.