allow config files to be specified using argparse

[MoralCode]

as GeoCompare is meant to be used as a command-line program, being able to pass in a different filename using --config parameter to allow reading a config file from a custom file/path would be nice to have.

This shoulld be relatively simple and should just involve:

1. uncommenting the import argparse line in main.py
2. looking at the extensive documentation for argparse
3. adding the correct method calls to define and parse arguments immediately after if __name__ == "__main__":
4. update the configfile = Config.from_file('./config.json') line to use the values from argparse instead of a hardcoded filename
5. set the old hardcoded filename as the default value for argparse so that the default config file is used if a custom one is not specified