add beforeProxy option to handle request before send to proxy

https://github.com/spmjs/spm-server/issues/4
改成可配置? follow  的话不如现在用不了，会把 html 合并回来。

```bash
$ curl -I https://a.alipayobjects.com/b.js
HTTP/1.1 302 Found
Server: Tengine
Date: Thu, 14 Aug 2014 04:51:51 GMT
Content-Type: text/html
Content-Length: 258
Connection: keep-alive
Location: http://err.taobao.com/error1.html
Expires: Fri, 14 Aug 2015 04:51:51 GMT
Cache-Control: max-age=31536000
```