Execution example::

  select Documents \
    --output_columns 'snippet(content, \
                              { \
                                 "default_open_tag": "[", \
                                 "default_close_tag": "]", \
                                 "width": 50 \
                              })' \
    --match_columns content \
    --query "fast performance"
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     [
  #       [
  #         1
  #       ], 
  #       [
  #         [
  #           "snippet", 
  #           null
  #         ]
  #       ], 
  #       [
  #         [
  #           "Groonga is a [fast] and accurate full text search en", 
  #           " result in superior [performance] on real-time appli"
  #         ]
  #       ]
  #     ]
  #   ]
  # ]
