Json config loader class
It appears that the file ConfigLoader.cpp is unnecessary unless you are just providing an example of usage.
The keyword inline is obsolete, it is really only a suggestion these days. Modern C++ compilers will make the decision on whether the code should be inlined or not when using the -O3 switch.
If you are going to let the user define the config file name, then pass the name into the constructor and default it at a higher level.