Enable config manipulation after parsing

[qzed]

Provide a way to safely (if possible without direct downcasting) manipulate configurations after parsing. Possible use-case: Transform relative into absolute Target-paths.

[qzed]

c4bc3d5 adds visitor-functions which simplify editing Config trait-objects. The whole Target-path-changing-situation could be implemented directly on top of that, however, I don't think this is a good approach (e.g. it's quite easy to miss a visitor for some type). A solution to this should provide direct support for the built-in defaults and a structure that does not make it too easy to miss a visitor-implementation for a custom-defined type.