Add a way to merge Stryker config files

[psfinaki]

Is your feature request related to a problem? Please describe.
In large repos, different projects might be built for different frameworks and their intersection is not guaranteed. In that case, the "default" framework in the Stryker config makes sense but it would be helpful to be able to override that default for certain folders (e.g. dedicated to some analyzers, generators, etc) instead of relying on the fallback mechanism.

Describe the solution you'd like
The idea is to follow the MSBuild approach in that one would be able to specify a Stryker config on different levels in the repo and the settings in the innermost config would override the outermore (thus similar to Directory.Build.Props).

[dupdob]

Note that while I redesigned solution mode, I deliberately left the design open so that options could be different on a per project basis. Whats remains to be done is:

1. Loading per project options (if/when present)
2. Implement the inheritance strategy