Decorate a Feature method to register it as an output formatter.

    All formatters are picked up by the argument parser so that they can be
    listed and selected on the CLI via the -f, --format argument.

def formatter(name, default=False):
    """Decorate a Feature method to register it as an output formatter.

    All formatters are picked up by the argument parser so that they can be
    listed and selected on the CLI via the -f, --format argument.
    """
    def decorator(func):
        func._output_format = dict(name=name, default=default)
        return func
    return decorator