MS Visual Studio "/Zc:wchar_t-" compiler parameter support added

[slavanap]

If /Zc:wchar_t- compiler parameter is specified, then wchar_t is defined as
typedef unsigned short wchar_t;
Thus, this leads to conflict in template definition in format.h

You can verify it in any Visual Studio project with spdlog with

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /Zc:wchar_t-")

reference: https://msdn.microsoft.com/ru-ru/library/dh8che7s.aspx

[vitaut]

Merged with a minor change in acfa858 (without this change unsigned short values would be formatted as characters). Thanks!