- Disable templated user-defined literals when compiled under nvcc

Similarly to Intel's C++ compiler, nvcc fails to compile templated UDLs
as they both share the same parsing engine (EDG C++ Front End).  This
fixes building CUDA programs which make use of fmt.

Changed "reusing existing formatters example" to use scoped enum instead.

Get rid of conversion warning in gcc-4.8 

Get rid of the following warning:
conversion to ‘int’ from ‘long unsigned int’ may alter its value [-Werror=conversion]

This change enables FMT_FALLLTHROUGH also for GCC-Compilers in the versions 7 and 8.