title: Vuelidate: validate form with sub components
body: <p>How to work with validations of nested components inside a parent component with <code>Vuelidate</code>? I would like to change <code>parentForm.$invalid</code> if inputs in subcomponents are valid or not.</p>
  
  <p>Parent:</p>
  
  <pre><code>&lt;parent-component&gt;
    &lt;/child-component-1&gt;
    &lt;/child-component-2&gt;
  &lt;/parent-component&gt;
  
  validations: {
    parent: WHAT HERE?
  }
  </code></pre>
  
  <p>Child-1</p>
  
  <pre><code>&lt;child-component-1&gt;
    &lt;/some-input&gt;
  &lt;/child-component-1&gt;
  
  data() {
    return {
      someInput: ""
    };
  },
  
  validations: {
    someInput: required
  }
  </code></pre>
  
  <p>Child-2</p>
  
  <pre><code>&lt;child-component-2&gt;
    &lt;/some-input&gt;
  &lt;/child-component-2&gt;
  
  data() {
    return {
      someInput: ""
    };
  },
  
  validations: {
    someInput: required
  }
  </code></pre>