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.