stderr in ffmpeg_write redirected (file/DEVNULL). Cleaned docs.

Now the stderr in ffmpeg_write is redirected towards DEVNULL
or towards a logfile, as an option.
Now the default pixel format for mp4 (libx264) is yuv240p UNLESS the
wideo has a dimension that is odd (non-even), at which case the
default pixel format will be, well, the default, which may cause
trouble on many readers.
Cleaned up the docs and added a FAQ and links to the reddit forum
and the librelist.
removed debug print statement