Allow for user extension

#1

It would really be helpful to have some extension points for postman.

For example: I’ve got a OpenAPI document that is describing my API. Now after a request was done, I can lookup the done request and check that against the JSON schema from the OpenAPI document. I also can get the response code and check the the schema of that response code. For all of this I currently have code, but I need to insert it on each request doing some eval of a environment variable. I’d like to have a cleaner way.