Add zf_log to performance tests

On my machine I'm getting following results (real time, best of 3):
        1 thread   10 threads   100 threads
zf_log   0.466s     2.711s       2.608s
spdlog   0.747s     5.121s       5.376s

bench/zf_log-bench-mt.cpp

@@ -0,0 +1,56 @@
+#include <thread>
+#include <vector>
+#include <atomic>
+#include <cstdlib>
+#include <unistd.h>
+#include <fcntl.h>
+#include <zf_log.c>
+
+const char g_path[] = "logs/zf_log.txt";
+int g_fd;
+
+static void output_callback(zf_log_message *msg)
+{
+	*msg->p = '\n';
+	write(g_fd, msg->buf, msg->p - msg->buf + 1);
+}
+
+using namespace std;
+
+int main(int argc, char* argv[])
+{
+	g_fd = open(g_path, O_APPEND|O_CREAT|O_WRONLY);
+	if (0 > g_fd)
+	{
+		ZF_LOGE_AUX(ZF_LOG_STDERR, "Failed to open log file: %s", g_path);
+		return -1;
+	}
+	zf_log_set_output_callback(ZF_LOG_PUT_STD, output_callback);
+
+    int thread_count = 10;
+    if(argc > 1)
+        thread_count = std::atoi(argv[1]);
+    int howmany = 1000000;
+    std::atomic<int > msg_counter {0};
+    vector<thread> threads;
+
+    for (int t = 0; t < thread_count; ++t)
+    {
+        threads.push_back(std::thread([&]()
+        {
+            while (true)
+            {
+                int counter = ++msg_counter;
+                if (counter > howmany) break;
+				ZF_LOGI("zf_log message #%i: This is some text for your pleasure", counter);
+            }
+        }));
+    }
+
+    for (auto &t:threads)
+    {
+        t.join();
+    };
+	close(g_fd);
+    return 0;
+}