util.inspect: support for sparsed arrays

  Modify output format for util.inspect in such manner that allow compact output for a large
sparsed array.
   Was:
   [,,,,,,,,,,,,,,,5,,,,,...,,,,10,,,...,,,] (... mean
thousands commas)
   Now
   [15:5, 2000:10, 9999: ]

fixes:  joyent/node#6926