Method for configuring environment resolver.

        :param var_name: Variable name
        :param env_name: An optional environment variable name. If not set\
            haps looks for `HAPS_var_name`
        :param default: Default value for variable. If it's a callable,\
            is called before return. If not provided\
            :class:`~haps.exceptions.UnknownConfigVariable` is raised
        :return: :class:`~haps.config.Configuration` instance for easy\
                  chaining

def env_resolver(cls, var_name: str, env_name: str = None,
                     default: Any = _NONE) -> 'Configuration':
        """
        Method for configuring environment resolver.

        :param var_name: Variable name
        :param env_name: An optional environment variable name. If not set\
            haps looks for `HAPS_var_name`
        :param default: Default value for variable. If it's a callable,\
            is called before return. If not provided\
            :class:`~haps.exceptions.UnknownConfigVariable` is raised
        :return: :class:`~haps.config.Configuration` instance for easy\
                  chaining
        """

        cls.resolver(var_name)(
            partial(
                _env_resolver, var_name=var_name, env_name=env_name,
                default=default))
        return cls()