Set a new value for the specified key.

def set(file_name, key, value, new):
    """
    Set a new value for the specified key.
    """
    db = XonoticDB.load_path(file_name)
    if key not in db and not new:
        click.echo('Key %s is not found in the database' % key, file=sys.stderr)
        sys.exit(1)
    else:
        db[key] = value
        db.save(file_name)