Don't overwrite environment variables that are already set #16
Description
In both node and ruby dotenv, it does not overwrite environment variables that are already set:
- https://github.com/bkeepers/dotenv#why-is-it-not-overriding-existing-env-variables
- https://github.com/motdotla/dotenv#what-happens-to-environment-variables-that-were-already-set
This library should emulate that expectation.