Extract incremental build logic into separate classes

[ashmaroli]

• This is a 🔨 code refactoring.

Summary

The major methods of Jekyll::Regenerator check if site.config["incremental"] to determine course of action. This is a code-smell.
Therefore, stop such checks on every call and extract the incremental logic into a new subclass.
Additionally, subclass Jekyll::Site to use the new Regenerator subclass.