Investigate using a single yml file for managing regex

[stephanmiehe]

Currently versions are broken out into dedicated yml files. This can lead to duplicates between versions. Explore using a single file and making iterating on rules simpler without the need to duplicate information.

[keikoro]

Relatedly, I was thinking it would be useful to be able to add onto (or subtract from) an existing "top level" configuration-path set in a reusable workflow.

This would allow the use of a shared set of rules (for shared labels) across repositories, similar to what #39 was concerned with, but then let individual repos adapt/extend the default config in a DRY manner. E.g. to add rules for additional labels only present in the calling repos, or to disable or overwrite existing rules.