title: Update Validators in FormControl
body: <p>is there any way to update control after it declared, like</p>
  
  <pre><code>this.input = new FormControl('', Validators.required)
  this.form = this.formBuilder.group({
    input = this.input
  })
  
  this.input.update('', Validators.maxlength(20))
  </code></pre>