title: Unset all properties of a Backbone.js model that start with the same substring
body: <p>I would like to unset all properties of a Backbone.js model that start with the same substring like:</p>
  
  <pre><code>logo_id
  logo_width
  logo_height
  </code></pre>
  
  <p>I would like to do something like:</p>
  
  <pre><code>model.unsetAllStartsWith('logo');
  </code></pre>
  
  <p>The code would be something like:</p>
  
  <pre><code>_.extend(Backbone.Model, {
      unsetAllStartsWith : function(str){
          // something with this.toJSON() ?
      }
  });
  </code></pre>
  
  <p>I get confused with models and toJSON.  Also, is that the correct way to extend the Backbone model class?</p>