Support providing a log level to empire

[mwildehahn]

This would let us add more debug logging via logger.Debug(ctx, ...) and only enable these during dev or debugging.

[ejholmes]

Somewhat related, I'd love to do a revamp of logging. There's some places where we use the go logger (e.g. cmd/empire) and some places where we use pkg/logger. It'd also probably be good to remove the dependency on pkg/logger and bring it in as remind101/empire/logger.