Add ability to register additional extractors in extractor map

[jacospain]

It would be convenient to be able to register additional extractors into the config module.

In particular, this would be nice for using this library in Scala projects, where often the consuming code can more naturally be expressed using a Scala type instead of the Java types injected by this library (e.g. scala.concurrent.duration.Duration instead of java.Time.Duration, a scala collection instead of java.util.List).