[Swftools-common] Support for extracting embedded MP3s

[Nathan Summers]

I had a SWF with a sound stream embedded in a sprite.  According to the
man page for swfextract, the way to extract the sound stream is to use the -i
to extract a MovieClip and then to use -m on the extracted .swf.  I could not
get that to work.  This patch allows you to use swfextract with a new option,
-M, to extract sound streams from within sprites without having to extract the
sprite first.  With this patch I was able to extract the sound stream
successfully.