Add --host and --port command-line parameters to app.py

[IntegralProgrammer]

Adds the --host and --port command-line arguments to app.py so that it can listen on a specific interface and on a specific port. If --host is not specified it defaults to 127.0.0.1. If --port is not specified it defaults to 5000. Therefore if neither --host nor --port are specified, no new behavior is introduced to app.py as it, just like before, listens for requests on 127.0.0.1:5000.