alllow the source and target datasets to be specified by name using argparse arguments

see #15 for more details on getting started with argparse and how/where to implement it.

essentially, the basic idea of this issue should be to allow the names of two sources from `config.json` to be specified by their name to be used for comparison. i.e. `python3 main.py --source al-arcgis-vaccination-sites --target giscorps-vaccination-sites`