Deciding on a standard, default convention for configuration

https://github.com/mehcode/config-rs/issues/7

Looking for feedback. I'm looking to make the config crate more ergonomic to "sit down and use" and I'm thinking this is a good way to go about it.

The general idea is a set of default "configuration sources" (can be disabled with a feature flag or lack of one) that set up a "recommended" arrangement of configuration.

Cross-posted from https://www.reddit.com/r/rust/comments/5t53xf/deciding_on_a_standard_default_convention_for/ to increase visibility.