Add a way to pass arbitrary arguments to ffmpeg.run()

Great work!

I’d like to pass -loglevel quiet to ffmpeg.run. I’ve tried this but it fails:

>>> stream = ffmpeg.nodes.GlobalNode(stream, 'loglevel', 'quiet')
>>> ffmpeg.run(stream)
AssertionError: Unsupported global node: loglevel(quiet)

I’ve used ffmpeg.run(stream, cmd=['ffmpeg', '-loglevel', 'quiet']) as a workaround, but it looks like GlobalNode is very restricted. Also multiple GlobalNode can’t be chained.

[roman-ku]

For anybody that needs an example (like I did):

(
    ffmpeg
    .input(in_filename, ss=time)
    .output(out_filename, vframes=1)
    .global_args('-loglevel', 'error')
    .global_args('-y')
    .run()
)

[Hodapp87]

I’m likewise running into the issue that I can’t use this when I need an argument to be before all else, e.g. -hwaccel.