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

Created by: elelayan

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.