network_traffic: allow to hide down interfaces

Interfaces which are down can be hidden or formatted differently.

The following options are added:
hide_down - whether to not display a interface which is down
format_down - format string if the interface is down (unless hide_down is set)
network_traffic: do not hide by default down iface
Merge pull request #138 from simon04/network_traffic_down

network_traffic: allow to hide down interfaces