title: i18n Translate.instant with parameters
body: <p>I need to use TranslateService.instant from ngx-core and pass parameters to do string interpolation.<br>
  So far I've done:</p>
  
  <pre><code>MY_STRING: "Welcome {{user}}"
  </code></pre>
  
  <p>In my component I do:</p>
  
  <pre><code>translateService.instant(MY_STRING, {user: 'Nick'})
  </code></pre>
  
  <p>and what I get from this is:</p>
  
  <pre><code>Welcome {{user}}
  </code></pre>
  
  <p>The parameter user is not interpolated. What am I doing wrong?</p>