Added convenience macro.

examples/demo.cpp

@@ -15,7 +15,7 @@ int convert<int>(const std::string &value) {
 
 struct foo : settable::settable {
     foo() {
-        register_setter<int>("x", &x);
+        REGISTER_MEMBER(int, x);
     }
 
     int x;

settable.hpp

@@ -52,6 +52,9 @@ public:
     void *ptr;
 };
 
+#define REGISTER_MEMBER(Type, Attribute) \
+    this->register_setter<Type>(#Attribute, &Attribute);
+
 class settable {
 public:
     void set_attribute(const std::string &attr, const std::string &value) {