Support directly generating Postman Collections JSON

It would be useful to be able to export directly to Postman collections format (v2 - structure documented here).

There are other existing JS libraries to do this (e.g. https://github.com/javieraviles/Har2Postman/) and it's very simple (it's a JSON -> JSON transformation) but integrating that in Httpsnippet too would be useful, to make it available within the same structure & using the same API as all the other HTTP export formats here.

首先查看 Httpsnippet 现有的 HTTP 导出格式实现及其使用的共享 API，然后将所需输出与 Postman Collections v2.1 架构和链接的 Har2Postman 库进行比较。完成标准是通过与其他格式相同的 API 直接生成有效的 Postman Collections JSON。