added listen-port option for temp python server

[koyae]

Since port 80 is a privileged port, regular applications cannot bind to it (that is, listen to / read traffic from it) by default under Linux. Rather than granting special rights to allow this, some servers simply route traffic on to other ports (such as 8080) after initially being received on port 80.

This change allows the listening port used in the non-file-based mode to be configured as needed.