Loaders Should Return {key, value} Instead of Handling Themselves

At the moment, both the Environment and File loaders apply the values they find, returning keys they apply.

This means that each loader is responsible for sanitizing and filtering the keys (only: []) that should be loaded.

I'm proposing that the loaders responsibility should be tweaked to simply return a tuple with the key and value found, and another step within loader.ex should handle applying; which also allows us to warn on duplicate keys within multiple loaders.