Migrate config file merging to nlohmann

I'm moving the file fragment merging to use nlohmann::json rather than ptree. I've also modified test-config-merge to dump the merged json file, since I'm removing the ptree-based merging.

This starts to address the overall migration to nlohmann::json (#52 (closed)), at least at the top level. I'm breaking this into parts so I can work on the configuration file restructuring in #51 (closed) now: I didn't want to migrate everything to another parser just to change the structure, and I also didn't want to change the structure in ptree and then migrate everything to nlohmann.