Added async_nonblocking factory

include/spdlog/async_logger.h

@@ -23,6 +23,14 @@
 #include <string>
 
 namespace spdlog {
+
+// Async overflow policy - block by default.
+enum class async_overflow_policy
+{
+    block,         // Block until message can be enqueued
+    overrun_oldest // Discard oldest message in the queue if full when trying to add new item.
+};
+
 namespace details {
 class thread_pool;
 }