return only parsed keys/values

This PR is to return the parsed keys/values as those can be useful for meta programs that need the list of configured keys.

Fortunately, if the caller wants to continue with the old behavior, they simply reference `process.env`.

Not sure I love the use of `_extend`; however, I doubt the API will change.

Now that I've seen `params`, I'm starting to think I can use it to get rid of the `forEach`. Hold on a sec... :)