Supporting github enterprise

Of all the github api libs, I find this one to be the most well written. I've been using it for working with gihub enterprise and it works fine for most gets.

However, it fails on POST/PUT where:
- SSL errors crop up due to enterprise being signed by an internal certificate signing authority
- no ability to pass extra headers

Above 2 can be solved if there was a way to pass on additional headers for the request. Currently I'm using request-promise and passing the CA chain as well as custom headers when POST/PUT are needed but this is sub optimal solution.

I'm using this in a Node (v4.4.3 and above) app.

未指定文件或测试。首先跟踪 POST 和 PUT 请求是如何构造的，然后检查现有的 request-promise 使用方式，以及请求选项如何携带 CA 链和其他标头；完成的标准是 Enterprise 的 POST/PUT 请求可以同时使用二者，而不会出现报告中的 SSL 错误。

由索引模型根据 Issue 内容生成。