[modules] Add support for disk utilization

Add a new module "disk" that takes an optional parameter (the path) and
displays free & total disk space, along with the usage percentage.

Also, added Tunnel/VPN support to the themeing of the "net" module.

bumblebee/themes/solarized-powerline.json

@@ -32,6 +32,9 @@
 	"cpu": {
 		"prefix": "  "
 	},
+	"disk": {
+		"prefix": "  "
+	},
 	"nic": {
 		"states": {
 			"wireless-up": {
@@ -45,6 +48,12 @@
 			},
 			"wired-down": {
 				"prefix": "  "
+			},
+			"tunnel-up": {
+				"prefix": "  "
+			},
+			"tunnel-down": {
+				"prefix": "  "
 			}
 		}
 	},