Add flag option for config.json file

Add a flag option (-c ?) and possibly an environment variable to allow specifying the location of the config.json file. This would allow execution in different directories while utilizing the same configuration.

Start by locating the CLI argument parsing and the code that loads config.json. Implement a -c option, and consider the requested environment-variable alternative, so execution from different directories can reuse one configuration; done means the selected configuration location is honored.

Written by the indexing model from the issue text.