"external" configurations should be merged

[rhuss]

Although technical probably a bit involved, from a usage perspective it regular configuration and configuration from external sources should be merged. Of course, things could become a bit complicated to debug, but it allows for very flexible use cases.

Some things to consider:

• Merging should respect the order of the declaration (although I don't know whether this is possible at all with Maven to repeat e.g. <external> snippets in arbitrary order). Maybe an order parameter is needed.
• The name "external" should be reconsidered.
• In debug mode, a view of the the complete and merged configuration should be possible.