Add support for multi-output filters; implement `split` filter

The split operator now does the right thing:

import ffmpeg

overlay_file = ffmpeg.input('overlay.png')
split = (ffmpeg
    .input('input.mp4')
    .hflip()
    .split()
)
(ffmpeg
    .concat(
        split[0].vflip(),
        split[1]
    )
    .output('out.mp4')
)

This paves the way to having both multi-input and multi-output components. To access particular outputs of a multi-output node, use either the .stream function or bracket shorthand:

split = in.split()
split0 = split.stream(0)
split1 = split[1]

I decided to forego the automatic split stuff for now. It was probably more work to add multi-output support right away, but I think it's more correct in the long run. The automatic splitting will eventually happen as a pre-processing step and produce the same kind of graph as though the split were inserted manually.