[FR] Customize multicast address

[oakaigh]

@duncanthrax It is found that some routers have firewalls configured to block exotic multicast addresses, e.g. any multicast address other than 239.255.255.250 (UPnP). Plus, for the sake of consistency and maintainability, users should be liberal to choose whatever multicast address they want, pretty much in the same way they customize the unicast address.