drop nested config

This functionality was never actually used AFAICT, as no way to provide
alternate profiles was ever provided.

This changes the configuration format to remove the `[global]` section,
everything that was previously under that namespace is now at the top
level.

src/main.rs

@@ -111,7 +111,7 @@ async fn try_main() -> Result<(), error::Main> {
     let args = args::parse();
 
     // Initialize config
-    let raw_config = Figment::new().merge(Toml::file(&args.config).nested());
+    let raw_config = Figment::new().merge(Toml::file(&args.config));
 
     let config = raw_config.extract::<Config>()?;