Refactor configuration loading to have a defined schema

[matthiasr]

Use case. Why is this important?

This is the Big Solution follow up to #493. Instead of loading YAML into unstructured objects, we really ought to load into a structure that represents exactly the configuration file. This is also the approach in the Go-based exporters, because that's how YAML loading works in Go to begin with. It also helps with validating configuration, cf. #484.

How do you think the new configuration should look like?

This should not change the contents or semantics of the configuration.