weed shell: adding file system disk usage fs.du

fix https://github.com/chrislusf/seaweedfs/issues/889

echo "fs.du http://localhost:8888/some/path" | weed shell

weed/shell/shell_liner.go

@@ -36,6 +36,7 @@ func RunShell(options ShellOptions) {
 		env: make(map[string]string),
 		masterClient: wdclient.NewMasterClient(context.Background(),
 			options.GrpcDialOption, "shell", strings.Split(*options.Masters, ",")),
+		option: options,
 	}
 
 	go commandEnv.masterClient.KeepConnectedToMaster()