Regression in nested environment paths

[conradludgate]

In config 0.11, our app used an environment source

Environment::with_prefix("app").separator("__")

Which would match environment vars like APP_TLS__CERTIFICATE (into tls > certificate).

This no longer works due to the group separator using the specified separator, as opposed to always being _ https://github.com/mehcode/config-rs/blob/30d87a311929f5382cb4ecd1de62f20d2dddfb86/src/env.rs#L123

This forces me to update my environments to APP__TLS__CERTIFICATE which is not the end of the world but it slows adoption of the new version.

I'm not sure on the best solution, but maybe there could be a new prefix_separator field. This can be set automatically by the separator method too so that it doesn't break any existing 0.12 apps