Add an optional final qualifier to types

When building with GCC's -Wfinal-types, a lot of types of spdlog
are marked as being more optimizable if they were marked final.

This patch adds a possibility for the user of the library to `#define SPDLOG_FINAL final`
and enjoy potentially better performance : GCC is then able to replace virtual calls by true
function calls if it can ensure that there are no derived types).

By default SPDLOG_FINAL is defined to nothing to not break existing code that
may be inheriting of some of these types for some reason.

include/spdlog/async_logger.h

@@ -31,7 +31,7 @@ namespace details
 class async_log_helper;
 }
 
-class async_logger :public logger
+class async_logger SPDLOG_FINAL :public logger
 {
 public:
     template<class It>