added sout-keep option to keep sout instance across playlist
           items (usefull with 'gather' stream_out)
 * stream_output : implement the sout-keep mode.
 * playlist, video_output, stream_output: vout and sout are now reattached
 to playlist instead of being destroyed. There are automatically destroy
 if unused byt the new input (200 ms after access and demuxer have finished
 their inittialisation step.)