Change config assignment op to copy-and-swap

The previous implementation was not strongly exception safe.
This code is nearly identical logically, but with strong exception safety.
It's also good practice and more readable.

src/config.cpp

@@ -437,10 +437,8 @@ config& config::operator=(const config& cfg)
 	if(this == &cfg) {
 		return *this;
 	}
-
+	config tmp(cfg);
-	clear();
+	swap(tmp);
-	append_children(cfg);
-	values.insert(cfg.values.begin(), cfg.values.end());
 	return *this;
 }