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/commands.go

@@ -14,6 +14,7 @@ type ShellOptions struct {
 type commandEnv struct {
 	env          map[string]string
 	masterClient *wdclient.MasterClient
+	option       ShellOptions
 }
 
 type command interface {