added nlohmann_json library as submodule and to CMakeLists.txt
created basic config.json file in root folder
created ConfigLoader.h and added basic functionalities to it
After enter gets pressed on the start screen, a character selection screen gets displayed.
P1 can choose a character via 'a' and 'd' - P2 via 'arrow left' and 'arrow right'.
Selections get locked in upon pressing enter.
Code is currently working, but in need of improvements.