Feat: Reference Input from CLI

[zacharyabolton]

Reference Input from CLI

This change exposes the reference paramameter in the command-line interface
(CLI) for using reference based GEMBA scoring (methods that end with _ref) and
passes it downstream to pre-existing implementation for handling references.

Full Disclosure: I did have Claude Sonnet 4.5 help me write this change. However, I reviewed
every line of code and have tested it on three different datasets to ensure it
works as intended.

Defensive coding practices have been employed to validate the new reference
input and maintain backward compatibility with non-reference methods.

Thanks for considering my PR!

• Updates .gitignore
• Exposes reference parameter in CLI for using _ref methods