add other options on dealing with image streams

[maadjordan]

today i noticed that pdfsizeopt has the following error on test file:

D:\pdfsizeopt --v=20 --use-pngout=No --use-image-optimizer=ECT --use-multivalent=YES --do-fast-bilevel-images=Yes --tmp-dir=d:\temp "D:\test.pdf" "D:\test.pdf.pso"
Traceback (most recent call last):
  File "D:\Tools\Tiny\pdfsizeopt_win32exec\python26.zip\runpy.py", line 122, in _run_module_as_main
  File "D:\Tools\Tiny\pdfsizeopt_win32exec\python26.zip\runpy.py", line 34, in _run_code
  File "D:\Tools\Tiny\pdfsizeopt.single\__main__.py", line 1, in <module>
  File "D:\Tools\Tiny\pdfsizeopt.single\m.py", line 6, in <module>
  File "D:\Tools\Tiny\pdfsizeopt.single\pdfsizeopt\main.py", line 5630, in main
  File "D:\Tools\Tiny\pdfsizeopt.single\pdfsizeopt\main.py", line 4264, in OptimizeImages
  File "D:\Tools\Tiny\pdfsizeopt.single\pdfsizeopt\main.py", line 1701, in IsGrayColorSpace
  File "D:\Tools\Tiny\pdfsizeopt.single\pdfsizeopt\main.py", line 1753, in GetRgbPaletteSize
AssertionError: invalid palette size: 220

why not add option to skip such streams (so coping it as is to the new file) but highlighting that this document has this error or total errors found.