Logging

[barrucadu]

yukibot currently just dumps everything to stdout, which is sort of fine if you only have a single backend. But with multiple backends, multiple logs are really needed.

The logging has to be done at the backend level, as the bot gets a higher-level view, so bot-level logging would be useless for debugging.

[barrucadu]

Logging (both at the raw and the bot levels) is now implemented, but it would be nice to be able to log during the execution of plugins.