Issue 60: weed export -collection doesn't work

go/weed/export.go

@@ -82,6 +82,9 @@ func runExport(cmd *Command, args []string) bool {
 	}
 
 	fileName := strconv.Itoa(*exportVolumeId)
+	if *exportCollection!=""{
+	  fileName = *exportCollection + "_" + fileName
+	}
 	vid := storage.VolumeId(*exportVolumeId)
 	indexFile, err := os.OpenFile(path.Join(*exportVolumePath, fileName+".idx"), os.O_RDONLY, 0644)
 	if err != nil {