nixbot: Support different frontends

[infinisil]

Currently nixbot relies on a RabbitMQ frontend to do its messaging to IRC. This both limits the possibilities and introduces an unnecessary point of failure (I have no control over in this case). It would be nice to add different frontends with different capabilities:

• Pure frontend for testing
• IRC frontend
• RabbitMQ/AMQP frontend
• Console frontend for testing

Maybe even combine them in some ways, so I could e.g. log in as an admin into the bot from a console to control the IRC instance.