public function extendConfig( $container, $key, $default ) {
		$config = isset( $container[ WPEMERGE_CONFIG_KEY ] ) ? $container[ WPEMERGE_CONFIG_KEY ] : [];
		$config = Arr::get( $config, $key, $default );

		if ( $config !== $default && is_array( $config ) && is_array( $default ) ) {
			$config = array_replace_recursive( $default, $config );
		}

		$container[ WPEMERGE_CONFIG_KEY ] = array_merge(
			$container[ WPEMERGE_CONFIG_KEY ],
			[ $key => $config ]
		);
	}