Make it possible to override the item sorting

[kevinrenskers]

Currently all read items are sorted using their date: https://github.com/loopwerk/Saga/blob/main/Sources/Saga/ProcessingStep.swift#L114. But this isn't always what's wanted. Case in point: https://github.com/kevinrenskers/spamusement.cc. Here some of the comics have the same date, yet they have a very specific order: the ID within the frontmatter/metadata.

It would be nice if it was possible to tell Saga how to order the items.