cover discarded messages use-case

fix - formatter new discarded message

when using async_logger with async_overflow_policy::discard_log_msg each discarded
message will be counted and warning will be printed by the worker thread

this new feature is disabled by default - as it may have a performance hit when discarding messages

Allow compiler to select an strerror_r stringify

On Alpine (and potentially other systems) that don't identify their runtime correctly there is an issue with the string conversion
Specifically, alpine linux and musl where the errno_to_string is incorrectly called.
To fix this I have added two overloaded functions and use auto err to allow the compiler to detect the actual types returned and call the correct method for conversion

android sink - add retry mechanism

- in some cases subsequent calls to __android_log_write() may result with -EAGAIN error code.
  in such cases spdlog will sleep and try again for number of times defined by
  SPDLOG_ANDROID_LOG_NUM_OF_RETRIES

- defeult SPDLOG_ANDROID_LOG_NUM_OF_RETRIES set to 2 - can be overridden at build time

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.

Android_logger conditionally apply its own formatting

Android_logger apply its own formatting to every print by adding timestamps, severity, thread and process ids, thus the option flag in the ctor

Fix typo immediatly immediately

fix typos in base_sink.h

fix typo potentialy to potentially

Fix typo in comment Unkown to Unknown

Fix typo Unkown to Unknown

Update example in README.md

[#404] Reading past valid address with multisink logger

- Initialising atomic value

See examples at http://stackoverflow.com/q/36320008/706456
This issue was discovered with dr memory tool on Windows platform, Visual Studio 2015 C++ 11

- Initialising atomic value

See examples at http://stackoverflow.com/q/36320008/706456
This issue was discovered with dr memory tool on Windows platform, Visual Studio 2015 C++ 11