Add distributional sink

Setting up dist_sink to a logger will inject a layer between the logger and
(some of) it's sinks. Sinks can be added and removed to dist_sink dynamically.

include/spdlog/sinks/dist_sink.h

+/*************************************************************************/
+/* spdlog - an extremely fast and easy to use c++11 logging library.     */
+/* Copyright (c) 2015 David Schury.                                      */
+/* Copyright (c) 2014 Gabi Melman.                                       */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#pragma once
+
+#include <algorithm>
+#include <memory>
+#include <mutex>
+#include <list>
+
+#include "../details/log_msg.h"
+#include "../details/null_mutex.h"
+#include "./base_sink.h"
+#include "./sink.h"
+
+namespace spdlog
+{
+namespace sinks
+{
+template<class Mutex>
+class dist_sink: public base_sink<Mutex>
+{
+public:
+    explicit dist_sink() :_sinks() {}
+    dist_sink(const dist_sink&) = delete;
+    dist_sink& operator=(const dist_sink&) = delete;
+    virtual ~dist_sink() = default;
+
+protected:
+    void _sink_it(const details::log_msg& msg) override
+    {
+        for (auto iter = _sinks.begin(); iter != _sinks.end(); iter++)
+            (*iter)->log(msg);
+    }
+
+    std::vector<std::shared_ptr<sink>> _sinks;
+
+public:
+    void flush() override
+    {
+        for (auto iter = _sinks.begin(); iter != _sinks.end(); iter++)
+            (*iter)->flush();
+    }
+
+    void add_sink(std::shared_ptr<sink> sink)
+    {
+        std::lock_guard<Mutex> lock(base_sink<Mutex>::_mutex);
+        if (sink &&
+            _sinks.end() == std::find(_sinks.begin(), _sinks.end(), sink))
+        {
+            _sinks.push_back(sink);
+        }
+    }
+
+    void remove_sink(std::shared_ptr<sink> sink)
+    {
+        std::lock_guard<Mutex> lock(base_sink<Mutex>::_mutex);
+        auto pos = std::find(_sinks.begin(), _sinks.end(), sink);
+        if (pos != _sinks.end())
+        {
+            _sinks.erase(pos);
+        }
+    }
+};
+
+typedef dist_sink<std::mutex> dist_sink_mt;
+typedef dist_sink<details::null_mutex> dist_sink_st;
+}
+}