title: meaning of :: in angular formly
body: <p>i'm trying to learn how to work with angular firmly and i'm having trouble understanding some of the syntax used in the guides and examples on the <a href="http://angular-formly.com/#/" rel="noreferrer">official website</a>.
  when defining a button form control i saw this template:</p>
  
  <pre><code>&lt;div&gt;&lt;button type="{{::to.type}}" class="btn btn-{{::to.btnType}}" ng-click="onClick($event)"&gt;{{to.text}}&lt;/button&gt;&lt;/div&gt;
  </code></pre>
  
  <p>my question is: what is the meaning of "::" before the "to.type" and "to.btnType"?
  how is it being used?
  how is that different from defining it like this:</p>
  
  <pre><code>&lt;a ng-class="{'btn-primary': to.isPrimary, active: to.isActive}" class="btn, btn-default"/&gt;
  </code></pre>