[Feature]: add_hn_links_using_hack for any rss feed

[Albatrosicks]

Feature Summary

add_hn_links_using_opener and add_hn_links_using_hack content rewrite rules cannot be used, for example, for lobste.rs. Users need to be allowed to specify the base URL of the site for parsing comments.

What problem does this feature solve?

The ability to parse comments on websites such as lobste.rs

Proposed Solution

Allow optionally specifying the base URL for these functions

Alternatives Considered

No response

Additional Context

No response

Checklist

• I have searched existing issues to ensure this feature hasn't been requested before.
• I understand that feature requests are not guaranteed to be implemented.
• I agree to follow the project's contribution guidelines.