New Environment method to retrieve variables without removing the prefix

First of all, I really like the new version 0.12 of config-rs. Thanks for the great work, and continue like that!

I have few cases were I would like to use the following code:

    let settings = Config::builder()
        // Add in settings from the environment (with a prefix of APP)
        // Eg.. `APP_DEBUG=1 ./target/app` would set the `app.debug` key
        .add_source(config::Environment::with("APP"))
        .build()
        .unwrap();

For now, as I can see in the documentation and the examples, there is just an API to retrieve environment variables and this same API removes the current prefix. What I would like is to keep the prefix here.

Start at the Environment::with API and the documentation/examples showing environment-variable prefix handling. Define the requested behavior for retaining the prefix, then add coverage for prefixed variables and verify that the resulting keys match the issue's example.