Use TOML output for `-print-config` instead of JSON.

This is much easier to read, and can be used as a template for
a new configuration.

src/config.go

···
 	SlowResponseThreshold Duration `toml:"slow-response-threshold" default:"500ms"`
 }
 
-func (config *Config) DebugJSON() string {
-	result, err := json.MarshalIndent(config, "", "  ")
+func (config *Config) TOML() string {
+	result, err := toml.Marshal(config)
 	if err != nil {
 		panic(err)
 	}

src/main.go

···
 	}
 
 	if *printConfig {
-		fmt.Println(config.DebugJSON())
+		fmt.Println(config.TOML())
 		return
 	}