Added a config option for pup to allow setting the bound network interface

[shamada-kuuluu]

When having multiple network interfaces in a server, it can be necessary to be able to set which interface to bind to.
In my case, just binding to all interfaces was not an option.

This simple update to pup allows setting this. (Example config file is updated as well)

[shamada-kuuluu]

I am pretty sure, that the travis build failed due to other problems not connected with my changes.

[alq666]

There's more than just pup listening to an interface (dogstatsd among others) so we'd like to have a uniform way to configure which interface it binds to. Is the interface parameter meant to be an IP address?

[shamada-kuuluu]

Of course it makes sense to add this for the other services as well, but I focused on pup. Nevertheless I'd prefer an option for each service. That way you have fine grained control over what's listening where.
The interface parameter is usually an ip address, but it should work with a hostname as well (after all, the default is the hostname 'localhost' and not 127.0.0.1).