apply modern-cpp-template to the lib

[Gerodote]

applied this https://github.com/filipdutescu/modern-cpp-template

because current cmake script at the project literally sucks.

Please, do not lie about that this can be not header-only lib. It IS header only lib. Maybe it does precompiling headers ( including header to cpp and nothing to do), but in case of the lib, where there are a lot of templates, it's nearly useless.

Also removed test that was checking ability of linking the library, because it's pointless, if there's more useful test which needs the lib. If those fails, we can see it in compilation failing at preprocessing stage.

Edited README with adding explanation how to use the lib.

I didn't changed the code of the lib, except in fmtlog.h it adds fmtlog-inl.h anytime, because it's actually a header-only lib and let's not say it can be not header only just by adding one header to a .cc file.