Use extends attribute

[kbarbounakis]

A JSON model schema should have an option to extend a module given in an extends attribute e.g.

{
 "$schema": "https://themost-framework.github.io/themost/models/2018/2/schema.json",
  "extends": "./server/config/extends/Action.json",
  "name": "Action"
}

This operation will be very useful for prototyping schemas and use them across different environments without creating again.