Support multiple config files

[bunny-therapist]

Allow passing multiple config files. The config then gets merged, the same way it is merged with the args.

Since Configuration.merge already support merging multiple configs, this should be quite easy to implement.

Usecase: A group may want one config file for all their projects, plus an optional additional one per project, so that the group can have a list of vulnerabilities they have agreed on ignoring globally but then individual projects can ignore others.