Used argparse to parse the arguments

Resubmittion #3 : (hopefully correctly this time :D )

I used argparse to parse the arguments:

- The first argument (the path) is now optional, with the default value to set to the one imported from node. In the previous version the path imported was ignored and only the user-provided one was used.

- The second argument must be either 'upgrade' or 'downgrade', else it prints an error message (automatically).