Adds sar-based network-throughput widget

I've written a widget to show network throughput via the "sar" command. The output is formatted to display nicely no matter how much bandwidth the user may have. Styling is a personal preference so I imagine others will want to tweak that.

Note: I've written another throughput monitor that relies on netstat instead of sar, but it's less accurate. I can post somewhere if anyone is interested.