add new interface for init configuration (#15)

if_unix.go

@@ -0,0 +1,16 @@
+// +build linux darwin
+
+package water
+
+import "errors"
+
+func newDev(config Config) (ifce *Interface, err error) {
+	switch config.DeviceType {
+	case TUN:
+		return newTUN(config.Name)
+	case TAP:
+		return newTAP(config.Name)
+	default:
+		return nil, errors.New("unknown device type")
+	}
+}