ffmpeg: take bsf arguments from the command line

The format is now:
-bsf:X filter1[=opt1=str1/opt2=str2],filter2
ie the parameters are appended after the filter name using '='. As ','
has been reserved already for the list of filters, '/' is just an
example of token separation for now, but that could become part of the
API to avoid each bsf using its own tokenization.

The proper solution would be using AVOption, but this is overkill for now.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>