Possibility to bundle your own nlohmann/json

[hestad]

Example of spdlog giving the opportunity to bundle own fmt: https://github.com/gabime/spdlog/wiki/8.-Tweaking

///////////////////////////////////////////////////////////////////////////////
// Uncomment to use your own copy of the fmt library instead of spdlog's copy.
// In this case spdlog will try to include <fmt/format.h> so set your -I flag accordingly.
//
// #define SPDLOG_FMT_EXTERNAL
///////////////////////////////////////////////////////////////////////////////