Refactoring FriendlyId::Scoped::#slug_generator. Breaking it into two smaller methods

So (as you probably know) I have been looking into friendly_id codebase and bugging you guys for sometime. ;)

Anyway, while wanting to reuse some code I came across some code I felt would be helped by some refactoring. So I decided to submit this pull request. 

This refactoring: 
- Doesn't break any existing tests
- Breaks one method into two smaller once
- Keeps things consistent to how the `Slugged` module is coded
- Improves adherence to SRP 
- Will help me reuse some code if/when it goes live.