Using config-rs for layered configuration

Now that #109 has landed, I am looking to extend it so that there are other possible ways of setting up the execution environment for a Tide application. When it comes to application deployments, the [12-factor app](https://12factor.net/) idea is standard.

The [config-rs](https://docs.rs/config/0.9.2/config/) crate provides good support for layering configuration from different sources. It also seems to be the most downloaded crate in the configuration category according [crates.rs](https://crates.rs/config) (To be fair, that metric is pretty short sighted).

The issue is that it makes the API for accessing application configuration becomes a little verbose. I feel that the flexibility afforded is worth is.

What are your thoughts on this?