…" to the validators object that are passed into `Vue.use(VueFormGenerator, { validators: { key: (value, field, model) => {} })

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Feature

• What is the current behavior? (You can also link to an open issue here)
  VFG has a limited number of "built-in" validators, and custom validators require a reference to the function which is not possible when using JSON returned from the server.

• What is the new behavior (if this is a feature change)?
  Introduce the ability to modify the "built-in validators" when calling Vue.use(VueFormGenerator) by passing an options object with the validators.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information:
  Resolves Global custom validators #545

Example usage:

import Vue from "vue";
import VueFormGenerator from "../../../src";
Vue.use(VueFormGenerator, {
	validators: {
		customValidator: (value, field, model) => {
			console.log("customValidator", value, field, model);
			return true;
		}
	}
});

import App from "./app.vue";

new Vue({
	...App
}).$mount("#app");