Support zero index stream selectors

Hey 👋 I've just find out, that the library is not working as I expected in the case, when I tried to select stream with index 0. Examples:

ffmpeg.input('input.mp4').filter('scale', 360, -1).output('out.mp4').get_args()
# ['-i', 'input.mp4', '-filter_complex', '[0]scale=360:-1[s0]', '-map', '[s0]', 'out.mp4']

ffmpeg.input('input.mp4').node.stream(selector=1).filter('scale', 360, -1).output('out.mp4').get_args()
['-i', 'input.mp4', '-filter_complex', '[0:1]scale=360:-1[s0]', '-map', '[s0]', 'out.mp4']

Everything is ok, but:

ffmpeg.input('input.mp4').node.stream(selector=0).filter('scale', 360, -1).output('out.mp4').get_args()
# ['-i', 'input.mp4', '-filter_complex', '[0]scale=360:-1[s0]', '-map', '[s0]', 'out.mp4']

Instead of selecting specific stream, the whole input was selected. I think it should be:

ffmpeg.input('input.mp4').node.stream(selector=0).filter('scale', 360, -1).output('out.mp4').get_args()
# ['-i', 'input.mp4', '-filter_complex', '[0:0]scale=360:-1[s0]', '-map', '[s0]', 'out.mp4']