Execution example::

  select Documents --output_columns 'snippet_html("Groonga is very fast fulltext search engine.")' --command_version 2 --match_columns content --query "fast performance"
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         1
  #       ], 
  #       [
  #         [
  #           "snippet_html", 
  #           "null"
  #         ]
  #       ], 
  #       [
  #         [
  #           "Groonga is very <span class=\"keyword\">fast</span> fulltext search engine."
  #         ]
  #       ]
  #     ]
  #   ]
  # ]
