Add !nextfeature command

[Pengor]

Add a !nextfeature command that lets users vote for which feature they'd like implemented next

Command procedure should be:
User: !nextfeature
For each feature in the next feature set, the bot should respond:
Do you want to vote for feature #X: <issue title here>? (where X is the issue number)
User responds: X
Bot responds: Voted for issue #X
or
Vote changed to issue #X (if the user had previously voted

The bot should only list issues tagged as enhancements and not bugs. If their next response is not a valid issue number let them know they've exited the voting procedure.