use env if necessary in loading config (#409)

core/conf/options.go

@@ -0,0 +1,15 @@
+package conf
+
+type (
+	Option func(opt *options)
+
+	options struct {
+		env bool
+	}
+)
+
+func UseEnv() Option {
+	return func(opt *options) {
+		opt.env = true
+	}
+}