use specific outbound IP address

Hi,

I added changes mentioned in #14 (closed).

Although there is one side effect of this change, which may not be obvious on the first sight. If we specify public IP address localhost is out of scope/inaccessible - so for example in case you bind to public address you will have to rely on public name server and won't be able to use local one provided by systemd or alternative. (Unless of course it is bound to the public address as well)

Technically we could bypass this by bounding only sockets which do not target host machine. I am not yet sure how complex or what to change to achieve that, but if we want that as well I could look into that. Although it is not important for my scenario as I am targeting containers and isolation is a benefit for me.