add beforeProxy option to handle request before send to proxy

https://github.com/spmjs/spm-server/issues/4
发现我才是内网，cdn 的处理好奇怪

```
☖ curl -I https://a.alipayobjects.com/b.js
HTTP/1.1 404 Not Found
Server: spanner/1.0.6
Date: Thu, 14 Aug 2014 05:02:57 GMT
Content-Type: text/html
Content-Length: 590
Connection: keep-alive
```