TODOs

[bnjjj]

TODO:

• Better error report with explicit layer name
• Suggest crates like https://github.com/jonasbb/serde_with and add usecases
• Support Vault
• Implement a trait/api to extend and let users fetch config from remote
• Refactor to let user extend layers
• Add support of nested struct in envy
• Fix issue with #[serde(flatten)] when you use other type than String` in sub types
• Add support for other cli libs