fix golint issues in core/conf (#476)

core/conf/options.go

@@ -1,6 +1,7 @@
 package conf
 
 type (
+	// Option defines the method to customize the config options.
 	Option func(opt *options)
 
 	options struct {
@@ -8,6 +9,7 @@ type (
 	}
 )
 
+// UseEnv customizes the config to use environment variables.
 func UseEnv() Option {
 	return func(opt *options) {
 		opt.env = true