增加配置文件驱动接口，以便用户在遵循接口的前提下定义并使用自己的驱动，例如 YAML

library/think/config/driver/DriverInterface.php

@@ -0,0 +1,29 @@
+<?php
+// +----------------------------------------------------------------------
+// | ThinkPHP [ WE CAN DO IT JUST THINK ]
+// +----------------------------------------------------------------------
+// | Copyright (c) 2006~2015 http://thinkphp.cn All rights reserved.
+// +----------------------------------------------------------------------
+// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
+// +----------------------------------------------------------------------
+// | Author: dongww <dongww@gmail.com>
+// +----------------------------------------------------------------------
+
+namespace think\config\driver;
+
+/**
+ * 配置文件解析驱动接口
+ *
+ * @package think\config\driver
+ */
+interface DriverInterface
+{
+    /**
+     * 解析配置文件或内容，并以字符串形式返回
+     *
+     * @param string $config 配置文件路径或字符串
+     *
+     * @return array 以数组形式返回配置
+     */
+    public function parse($config);
+}

library/think/config/driver/ini.php

@@ -11,7 +11,7 @@
 
 namespace think\config\driver;
 
-class Ini
+class Ini implements DriverInterface
 {
     public function parse($config)
     {

library/think/config/driver/xml.php

@@ -11,7 +11,7 @@
 
 namespace think\config\driver;
 
-class Xml
+class Xml implements DriverInterface
 {
     public function parse($config)
     {