title: Angular 4 i18n for custom attributes
body: <p>Is there a way to translate custom attributes values in child-components using Angular i18n AOT?</p>
  
  <p>I know we can translate HTML element attributes as below.</p>
  
  <pre><code> &lt;input i18n-placeholder="search criteria date @@criteriaDate"
          placeholder="Date"
          formControlName="date" required&gt;
  </code></pre>
  
  <p>But I want to do the same thing for my component attributes. In this example I want to pass title attribute translated value.</p>
  
  <pre><code> &lt;custom-spinner
          formControlName="nights"
          [title]="'Nights'"
          i18n-title="search criteria nights@@criteriaNights"&gt;
        &lt;/custom-spinner&gt;
  </code></pre>
  
  <p>When I try this, it doesn't generate an entry on messages.xlf file.
  I couldn't find any examples on this.</p>