Support custom loggers

I'd like the option to use my own logger, like ```winston```.

There doesn't appear to be any behavior changes attached to ```config.debug```, and it's only used for logging purposes. To avoid breaking changes, I am using this field to also set custom loggers. Unless ```config.debug``` is a function, the logging will function exactly as it did before.

I would prefer to use a property that is a little more descriptive instead, like ```config.logging```, however that would not be backwards compatible.