title: Pass ng-model attribute to custom directive
body: <p>So, I have a form, where I need to use a custom directive.
  What i need: pass the <code>user</code> model to the directive. </p>
  
  <pre><code>&lt;form&gt;
      &lt;input type="text" ng-model="user.login"&gt;
      &lt;input type="password" ng-model="user.password"&gt;
  
      &lt;span ng-custom-directive ng-model="user.testfield"&gt;&lt;/span&gt;
  
  &lt;/form&gt;
  </code></pre>
  
  <p>Directive template looks like this:</p>
  
  <pre><code>&lt;span&gt;&lt;input type="checkbox" ng-model="[HERE I NEED user.testfield TO WORK WITH user]"&gt; &lt;/span&gt;
  </code></pre>
  
  <p>How I can pass the <code>user</code> model to directive template?</p>
  
  <p>After form submit I need <code>user.testfield</code> to be avaliable in the <code>$scope.user</code> like:</p>
  
  <pre><code>console.log($scope.user)
  {
      login: 'test',
      password: 'test',
      testfield: true|false
  }
  </code></pre>