Add a value to a key.

        Returns a list of warnings Conda may have emitted.

def config_add(self, key, value, **kwargs):
        """
        Add a value to a key.

        Returns a list of warnings Conda may have emitted.
        """
        cmd_list = ['config', '--add', key, value]
        cmd_list.extend(self._setup_config_from_kwargs(kwargs))

        return self._call_and_parse(
            cmd_list,
            abspath=kwargs.get('abspath', True),
            callback=lambda o, e: o.get('warnings', []))