Execution example::

  load --table Bookmarks
  [
  {"_key": "http://groonga.org/", "tags": ["groonga"]},
  {"_key": "http://mroonga.org/", "tags": ["mroonga", "mysql", "groonga"]},
  {"_key": "http://ranguba.org/", "tags": ["ruby", "groonga"]}
  ]
  # [[0,1337566253.89858,0.000355720520019531],3]
