slf4j-ize active-logger

- Split up `active-logger` into
- `active-logger-api` that uses the SLF4J facade as the "backend"
- and `active-logger-timbre` that uses `timbre-slf4j` as the backend and wraps timbre-specific stuff like configuration as `active-logger` does now
- Tie both together in `active-logger` in such a way that it transparently provides the same functionality as it does now

`active-logger` is then useful for stand-alone applications as it is now, `active-logger-api` is then useful for libraries.