title: ngx-translate default text if key is missing or translation file is being loaded
body: <p>I'm setting up a new Angular 7 app. I want to set a default text for translation. So in the translation {{ 'wait' | translate }}, I want to set text 'Waiting Now' as default text if there are any fallback. Means if data is being loaded or key is missing then passed value (in this case Waiting Now) should appear.</p>
  
  <p>I was trying to do something like <code>{{ 'Intro' | translate:'localizedText' }}</code></p>
  
  <p>Not didn't worked</p>
  
  <pre><code>{{ 'Intro' | translate:'localizedText' }}
  
  </code></pre>
  
  <p>I expect result should come as </p>
  
  <p>{{ 'Intro' | translate:'localizedText' }}
  => Intro (If being loaded or missing key)</p>
  
  <p>{{ 'Intro' | translate:'localizedText' }}
  => translated text</p>