Execution example::

  load --table WebClips
  [
  {"_key": "http://groonga.org", "content": "groonga is <span class='emphasize'>fast</span>"},
  {"_key": "http://mroonga.org", "content": "mroonga is <span class=\"emphasize\">fast</span>"},
  ]
  # [[0, 1337566253.89858, 0.000355720520019531], 2]
