Add graph visualization

Graph visualization is a thing now:

    split = (ffmpeg
        .input(TEST_INPUT_FILE)
        .vflip()
        .split()
    )
    split0 = split[0]
    split1 = split[1]

    overlay_file = ffmpeg.input(TEST_OVERLAY_FILE)
    return (ffmpeg
        .concat(
            split0.trim(start_frame=10, end_frame=20),
            split1.trim(start_frame=30, end_frame=40),
        )
        .overlay(overlay_file.hflip())
        .drawbox(50, 50, 120, 120, color='red', thickness=5)
        .output(TEST_OUTPUT_FILE)
        .overwrite_output()
        .view()
    )

graphviz must be installed in order for this to work, but if graphviz is unavailable .view() just throws an ImportError.