Added custom error handler support to async sink

include/spdlog/details/err_helper.h

@@ -18,6 +18,5 @@ public:
     void handle_unknown_ex(const std::string& origin, const source_loc& loc) const noexcept;
     void set_err_handler(err_handler handler);
 };
-
-
-}} // namespace spdlog::details
+} // namespace details
+} // namespace spdlog

include/spdlog/sinks/async_sink.h

@@ -42,7 +42,7 @@ public:
         err_handler custom_err_handler = nullptr;
     };
 
-    explicit async_sink(config async_config);
+    explicit async_sink(const config &async_config);
 
     // create an async_sink with one backend sink
     template <typename Sink, typename... SinkArgs>