Allow/disallow list

Thanks for this great extension, it's really helpful for me already!

I think it would be great to have an option for **allow/disallow lists**. Right now, I mostly see this being useful for personal websites or development projects and don't want to run it on every website. To define some kind of scope or lists would be a great addition!

The issue names no files, tests, or entry points. First inspect how the extension currently decides which websites receive it and how options are stored, then determine the expected precedence between allow and disallow lists. Done means users can configure a website scope and the extension consistently runs only where that configuration permits.

Written by the indexing model from the issue text.