Support directly generating Postman Collections JSON

[pimterry]

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.