[modules/traffic] Minor refactoring

* use psutil instead of "ifconfig" in order to avoid external command
calls
* fix a small bug in the ascii theme (missing colon)
* show statistics per-nic

themes/icons/ascii.json

@@ -62,9 +62,9 @@
 	},
 	"sensors": {
 		"prefix": "sensors"
-	}
+	},
 	"traffic": {
-		"down": { "prefix": "down"},
-		"up": {"prefix": "up"}
+		"rx": { "prefix": "down"},
+		"tx": {"prefix": "up"}
 	}
 }

themes/icons/awesome-fonts.json

@@ -80,7 +80,7 @@
     "prefix": "🌡"
   },
   "traffic":{
-	  "down": { "prefix": "" },
-	  "up": { "prefix": "" }
+	  "rx": { "prefix": "" },
+	  "tx": { "prefix": "" }
   }
 }