Disable output when debug is disabled

After a discussion with my teammate @fixe, we came to the conclusion that following tj's principles for the `debug` module it is unexpected to see `lugg` outputting logging from bunyan even when the env var `DEBUG` is not set. Since `v1.2.2`, bunyan also supports disabling logging by passing a higher level than `FATAL` to its options, so we think this is the best default approach for `lugg`.

This PR means that there is a breaking change and so the major version would have to be bumped if it is accepted. At the same time, the code readability has been improved, although that is a personal view on the matter.