Add option to display disk usage stats

[mroote]

I'm not sure if this is the best approach, but I think adding the ability to see current free space in the system would be a useful addition.

[davidscholberg]

Hey @mroote,

I think this functionality would be a great addition, but I'd like to see this use the text/template library like the iface module does. That way, you could specify {{.Status}} if you want the aggregate status (which should be the default to maintain backwards compatibility), or you could specify .AvailMB or {{.UsedGB/.TotalGB}} or whatever if you wanted to display the actual numbers.

Let me know what you think of this suggestions. Thanks!