Optimize encoding to sixel sequence.

http://mlterm.sf.net/optimize-sixel.png

This optimization works if -I option is specified.
'img2sixel -I snake' outputs:
  (normal) http://mlterm.sf.net/snake-fullcolor-3.six (825316bytes)
  (optimized) http://mlterm.sf.net/snake-fullcolor-4.six (802977bytes)