Validates that value is True, False, 'true', or 'false'.

def validate_boolean_or_string(option, value):
    """Validates that value is True, False, 'true', or 'false'."""
    if isinstance(value, string_type):
        if value not in ('true', 'false'):
            raise ValueError("The value of %s must be "
                             "'true' or 'false'" % (option,))
        return value == 'true'
    return validate_boolean(option, value)