Support input/output parameters

[kkroening]

This allows additional per-input/output settings to be specified, as in the following example:

out = (ffmpeg
    .input('pipe:0', format='rawvideo', pixel_format='rgb24', video_size=(width, height), framerate=10)
    .trim(start_frame=start_frame)
    .output('pipe:1', format='rawvideo')
)