Execution example::

  tokenize TokenBigram "Groonga is a fast fulltext earch engine!"
  # [
  #   [
  #     0, 
  #     1337566253.89858, 
  #     0.000355720520019531
  #   ], 
  #   [
  #     {
  #       "position": 0, 
  #       "force_prefix": false, 
  #       "value": "Gr"
  #     }, 
  #     {
  #       "position": 1, 
  #       "force_prefix": false, 
  #       "value": "ro"
  #     }, 
  #     {
  #       "position": 2, 
  #       "force_prefix": false, 
  #       "value": "oo"
  #     }, 
  #     {
  #       "position": 3, 
  #       "force_prefix": false, 
  #       "value": "on"
  #     }, 
  #     {
  #       "position": 4, 
  #       "force_prefix": false, 
  #       "value": "ng"
  #     }, 
  #     {
  #       "position": 5, 
  #       "force_prefix": false, 
  #       "value": "ga"
  #     }, 
  #     {
  #       "position": 6, 
  #       "force_prefix": false, 
  #       "value": "a "
  #     }, 
  #     {
  #       "position": 7, 
  #       "force_prefix": false, 
  #       "value": " i"
  #     }, 
  #     {
  #       "position": 8, 
  #       "force_prefix": false, 
  #       "value": "is"
  #     }, 
  #     {
  #       "position": 9, 
  #       "force_prefix": false, 
  #       "value": "s "
  #     }, 
  #     {
  #       "position": 10, 
  #       "force_prefix": false, 
  #       "value": " a"
  #     }, 
  #     {
  #       "position": 11, 
  #       "force_prefix": false, 
  #       "value": "a "
  #     }, 
  #     {
  #       "position": 12, 
  #       "force_prefix": false, 
  #       "value": " f"
  #     }, 
  #     {
  #       "position": 13, 
  #       "force_prefix": false, 
  #       "value": "fa"
  #     }, 
  #     {
  #       "position": 14, 
  #       "force_prefix": false, 
  #       "value": "as"
  #     }, 
  #     {
  #       "position": 15, 
  #       "force_prefix": false, 
  #       "value": "st"
  #     }, 
  #     {
  #       "position": 16, 
  #       "force_prefix": false, 
  #       "value": "t "
  #     }, 
  #     {
  #       "position": 17, 
  #       "force_prefix": false, 
  #       "value": " f"
  #     }, 
  #     {
  #       "position": 18, 
  #       "force_prefix": false, 
  #       "value": "fu"
  #     }, 
  #     {
  #       "position": 19, 
  #       "force_prefix": false, 
  #       "value": "ul"
  #     }, 
  #     {
  #       "position": 20, 
  #       "force_prefix": false, 
  #       "value": "ll"
  #     }, 
  #     {
  #       "position": 21, 
  #       "force_prefix": false, 
  #       "value": "lt"
  #     }, 
  #     {
  #       "position": 22, 
  #       "force_prefix": false, 
  #       "value": "te"
  #     }, 
  #     {
  #       "position": 23, 
  #       "force_prefix": false, 
  #       "value": "ex"
  #     }, 
  #     {
  #       "position": 24, 
  #       "force_prefix": false, 
  #       "value": "xt"
  #     }, 
  #     {
  #       "position": 25, 
  #       "force_prefix": false, 
  #       "value": "t "
  #     }, 
  #     {
  #       "position": 26, 
  #       "force_prefix": false, 
  #       "value": " e"
  #     }, 
  #     {
  #       "position": 27, 
  #       "force_prefix": false, 
  #       "value": "ea"
  #     }, 
  #     {
  #       "position": 28, 
  #       "force_prefix": false, 
  #       "value": "ar"
  #     }, 
  #     {
  #       "position": 29, 
  #       "force_prefix": false, 
  #       "value": "rc"
  #     }, 
  #     {
  #       "position": 30, 
  #       "force_prefix": false, 
  #       "value": "ch"
  #     }, 
  #     {
  #       "position": 31, 
  #       "force_prefix": false, 
  #       "value": "h "
  #     }, 
  #     {
  #       "position": 32, 
  #       "force_prefix": false, 
  #       "value": " e"
  #     }, 
  #     {
  #       "position": 33, 
  #       "force_prefix": false, 
  #       "value": "en"
  #     }, 
  #     {
  #       "position": 34, 
  #       "force_prefix": false, 
  #       "value": "ng"
  #     }, 
  #     {
  #       "position": 35, 
  #       "force_prefix": false, 
  #       "value": "gi"
  #     }, 
  #     {
  #       "position": 36, 
  #       "force_prefix": false, 
  #       "value": "in"
  #     }, 
  #     {
  #       "position": 37, 
  #       "force_prefix": false, 
  #       "value": "ne"
  #     }, 
  #     {
  #       "position": 38, 
  #       "force_prefix": false, 
  #       "value": "e!"
  #     }, 
  #     {
  #       "position": 39, 
  #       "force_prefix": false, 
  #       "value": "!"
  #     }
  #   ]
  # ]
