Allow HTTP port to be specified as argument

[nvlmn]

I've set up AirConnect (for airplay) and allowed the correct firewall ports, but after each restart, it picks a random port for HTTP, so it cannot be easily allowed through the firewall. Could a runtime argument that specifies the HTTP port get added?

[nvlmn]

Correction: I'll just add a pull request for the README adding that -a sets the HTTP port too, as I found out after looking at aircast.c.