Adding iface automatic detection for NetGraph (using /proc/net/route)

Hello again,

Since I use a laptop, I'm often moving from wireless to wired connection and back again, thus I needed my NetGraph widget to be able to update (relatively) smoothly.

The default interface is now "auto", which toggles automatic detection at widget initialization, using the default kernel route interface.

However, in order to change interfaces, you'll need to reload qtile. I'll probably add a cached method later, in order to update the detected interface on runtime.

Hope it'll be usefull !