[WIP] Validate hooks existence

Suppose `ConfigFile` holds a new configuration entry. Let's call it `navarro`, and say it's a boolean that indicates whether `ruby-git-hooks` should work without hair.

Now, `Configurations` has to explicitly delegate `#navarro` to `config_file`.

In other words, the knowledge of the existence of `navarro` as a configuration flag is now spread among two classes. Is that what you want?

I think it hurts DRY principle. In the case there's something I'm not seeing, please let me know.

A simple way around the issue I'm raising is making `Configurations` inherit from `SimpleDelegator` and change the constructor to include `super config_file`.