Remember loaded variables for later inquiry

Provide a Dotenv.loaded hash for convenience.

Example use case:

When running my project's test suite, I want to loop through all variables in .env,
and exclude them from VCR 'cassettes' with VCR's filter_sensitive_data method.
By looping through through Dotenv.loaded.keys, I can configure this once, and no
longer have to worry about environment-specific values polluting my test fixtures.

Any reactions or feedback on this idea?