feat: allow enabling bind struct with an option

Signed-off-by: Mark Sagi-Kazar <mark.sagikazar@gmail.com>

experimental.go

@@ -0,0 +1,8 @@
+package viper
+
+// ExperimentalBindStruct tells Viper to use the new bind struct feature.
+func ExperimentalBindStruct() Option {
+	return optionFunc(func(v *Viper) {
+		v.experimentalFinder = true
+	})
+}