Execution example::

  select Documents --output_columns "snippet_html(content)" --command_version 2 --match_columns content --query "fast performance"
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         1
  #       ], 
  #       [
  #         [
  #           "snippet_html", 
  #           null
  #         ]
  #       ], 
  #       [
  #         [
  #           "Groonga is a <span class=\"keyword\">fast</span> and accurate full text search engine based on inverted index. One of the characteristics of groonga is that a newly registered document instantly appears in search results. Also, gro", 
  #           "onga allows updates without read locks. These characteristics result in superior <span class=\"keyword\">performance</span> on real-time applications."
  #         ]
  #       ]
  #     ]
  #   ]
  # ]
