Allow binding variables to loggers

This is to allow custom arguments to be logged with every message
returned by a logger.

Use-case: in centralized logging, I want to know which application
generated every message.
It seems like this not so much a feature of the interface as a feature of `Logger`. And that there's no reason to ever change it once the `Logger` is created. As such, maybe just allow passing in the extra arguments to `Logger.__init__` instead of having an additional method?