[#42] Use same format pattern as log4j's ISO8601DateFormat

* https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html
* https://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/helpers/ISO8601DateFormat.html
The vars `example-config` and `merge-config` are not used since you later access them via the `timbre` alias.