Added command-line options for gateway IP

[lonkaars]

This pull request adds a command-line option to specify the gateway ip:

$ sudo python3 kickthemout.py -h
Usage: sudo python3 kickthemout.py [options]

Options:
  --version             show program's version number and exit
  -h, --help            show this help message and exit
  -p PACKETS, --packets=PACKETS
                        number of packets broadcasted per minute (default: 6)
  -s, --scan            perform a quick network scan and exit
  -a, --kick-all        perform attack on all online devices
  -t TARGETS, --target=TARGETS
                        specify target IP address(es) and perform attack
  -g GATEWAY, --gateway=GATEWAY
                        specify gateway IP address