More human readable format for configuration

[rexim]

Since we migrated to serde #209 It's actually pretty easy to switch formats. Candidates are:

• YAML https://crates.io/crates/serde_yaml
• TOML http://alexcrichton.com/toml-rs/toml/index.html
• Hjson https://github.com/hjson/hjson-rust