[models] generate interface with map attribute

[mrkpks]

Properties with additionalProperties should be generated as the keymap

Example INPUT:

myInterface:
  type: "object"
  properties:
    myName:
      type: "string"
    myId:
      type: "string"
    myMap:
      type: "object"
      additionalProperties:
        $ref: "#/definitions/someOtherInterface"

OUTPUT:

export interface MyInterface {
  myName: string;
  myId: string;
  myMap: {[key: string]: SomeOtherInterface};
}