postproc.py: update --ffmpeg-options default, update docstring
postproc.py: update --ffmpeg-options default, update docstring

sfepy/sfepy,rc/sfepy,sfepy/sfepy,vlukes/sfepy,BubuLK/sfepy,BubuLK/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,BubuLK/sfepy,vlukes/sfepy,rc/sfepy