Added "clone()" support to loggers

include/spdlog/async_logger.h

@@ -55,6 +55,8 @@ public:
     async_logger(std::string logger_name, sink_ptr single_sink, std::weak_ptr<details::thread_pool> tp,
         async_overflow_policy overflow_policy = async_overflow_policy::block);
 
+    std::shared_ptr<logger> clone(std::string new_name) override;
+
 protected:
     void sink_it_(details::log_msg &msg) override;
     void flush_() override;