Duplicate parameters can be set in kwargs with an iterator

For instance, add multiple streamid in the output can be done like this:

ffmpeg.input(url).output('video.mp4', streamid=['0:0x101', '1:0x102'])

will output this command line ffmpeg -i video.mp4 -streamid 0:0x101 -streamid 1:0x102 video.mp4

Related to #285 (closed)