Execution example::

  select Documents \
    --output_columns 'snippet(content, \
                              "fast", \
                              "performance", \
                              { \
                                 "default_open_tag": "[", \
                                 "default_close_tag": "]" \
                              })'
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         2
  #       ], 
  #       [
  #         [
  #           "snippet", 
  #           null
  #         ]
  #       ], 
  #       [
  #         [
  #           "Groonga is a [fast] 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 [performance] on real-time applications."
  #         ]
  #       ], 
  #       [
  #         null
  #       ]
  #     ]
  #   ]
  # ]
