Introducing concept of turbo filters a la logback

Cribbing the name "Turbo" [from logback](http://logback.qos.ch/manual/filters.html#TurboFilter), both for developer familiarity and because anything with turbo obviously makes it better.

The idea is to introduce filtering earlier into the logging decision such that creating the `slogger.Log` structure and corresponding call to `runtime.Caller()` can potentially be avoided. In environments with frequent slogging at `slogger.Level` below the level filter (e.g., frequent `DEBUG` statements), the performance benefit of not calling `runtime.Caller()` can be significant.