Execution example::

  load --table Comments
  [
  {"_key": 1, "content": "I'm using groonga too!"},
  {"_key": 2, "content": "I'm using groonga and mroonga!"},
  {"_key": 3, "content": "I'm using mroonga too!"}
  ]
  # [[0, 1337566253.89858, 0.000355720520019531], 3]
  load --table Articles
  [
  {"content": "Groonga is fast!", "comment": 1},
  {"content": "Groonga is useful!"},
  {"content": "Mroonga is fast!", "comment": 3}
  ]
  # [[0, 1337566253.89858, 0.000355720520019531], 3]
