Write a notebook to the file with given name

def writef(notebook, nb_file, fmt=None):
    """Write a notebook to the file with given name"""
    if nb_file == '-':
        write(notebook, sys.stdout, fmt)
        return

    _, ext = os.path.splitext(nb_file)
    fmt = copy(fmt or {})
    fmt = long_form_one_format(fmt, update={'extension': ext})

    create_prefix_dir(nb_file, fmt)

    with io.open(nb_file, 'w', encoding='utf-8') as stream:
        write(notebook, stream, fmt)