Feature: add json-schema validator postprocessor in http/scenario gun

[scrabblett]

Hi!

In my work I came across the need to compare the response from the server with the specified json schema.

To do this, I wrote my own implementation of this postprocessor and thought that it might be useful for other users.

For json-schema validation - i use this library: https://github.com/xeipuuv/gojsonschema