Improve network block about i3status-rust

[greshake]

@etrombly did the net block, maybe he wants to help you out :)

[etrombly]

Shouldn't be too hard to add the ip. I didn't add it originally because there isn't a standard for formatting across blocks, it's handled at the block level, so I was waiting to see if anything was created for that. @greshake if I send an empty vec back for the view function will the block not be drawn at all? If not I can just change the status.

[greshake]

Correct, it will not be displayed.
See util.rs:99:

        let widgets = block.view();
        if widgets.len() == 0 {
            continue;
        }

[atheriel]

@greshake @etrombly Any objection to my upstreaming some related changes to the Net block? It would add these features.

[etrombly]

I don't have a problem with it. Anything to improve it is welcome.

[greshake]

Sure go ahead