Adding -i <ifname> to specify network interface to manipulate inside container

Hi,

I found myself needing more than just one additional interface for my containers, because my network configuration of containers reflects a production system with several independent VLANs.

I modified pipework to allow the user to (optionally) set the container internal interface. The optional parameter is positional and has to follow after IFNAME on the command line.

The command line parsing is not very nice, but I tried to stay as close to the original syntax to not break any existing usage of pipework.

I'd be happy, if you accept my pull request.

Bye,
Lukas