title: angular 2 how to sum column ngFor
body: <p>trying to understand how to sum the column from table, every  is added with *ngFor ( like todo-list), i wanna sum one of column, how i can do it in angular 2 ? 
  So i have table like this: </p>
  
  <pre><code>&lt;tr *ngFor="let item of items"&gt;
      &lt;td&gt;{{item.num1 }}&lt;/td&gt;
      &lt;td&gt;{{item.num2}}&lt;/td&gt;
      &lt;td&gt;{{item.num3}}&lt;/td&gt;
      &lt;td&gt;{{item.num4}}&lt;/td&gt;
      &lt;td&gt;{{item.num5}}&lt;/td&gt;
  &lt;/tr&gt;
  </code></pre>
  
  <p>and I need to sum all that columns below like:</p>
  
  <pre><code>&lt;tr *ngFor="let item of items"&gt;
       &lt;td&gt;{{item.num1 }}&lt;/td&gt;
       &lt;td&gt;{{item.num2}}&lt;/td&gt;
       &lt;td&gt;{{item.num3}}&lt;/td&gt;
       &lt;td&gt;{{item.num4}}&lt;/td&gt;
       &lt;td&gt;{{item.num5}}&lt;/td&gt;
  &lt;/tr&gt;
  &lt;tr&gt;
       &lt;td&gt;SUM1&lt;/td&gt;
       &lt;td&gt;SUM2&lt;/td&gt;
       &lt;td&gt;SUM3&lt;/td&gt;
       &lt;td&gt;SUM4&lt;/td&gt;
       &lt;td&gt;SUM5&lt;/td&gt;
  &lt;/tr&gt;
  </code></pre>