Implement gokit/log's Logger interface in the SugaredLogger

[alex-laties]

I'd like to extend the SugaredLogger to implement go-kit/log's log.Logger interface.

This would make zap easier to integrate with codebases that already use go-kit/log and simplify migrating to zap down the road.

SugaredLogger seems like the correct place to implement this as there is already an expectation around loosely typed, but structured, logging.

[alex-laties]

https://github.com/alex-laties/gokitzap does the thing