title: Reactive forms valueChanges how to watch just 2 fields[Angular2]
body: <p>How can I watch just 2 fields of an Angular ReactiveForm? 
  I have a form like the following:</p>
  
  <pre><code>this.filter = this.fb.group({
              text: ['', []],
              bankCountry: ['', []],
              bankCity: ['', []]
          });
  </code></pre>
  
  <p>How can I watch for changes related to just the last 2 fields?</p>