title: Add custom route to viewsets.ModelViewSet
body: <p>In the docs there is the example of methods with custom url:
  <a href="http://www.django-rest-framework.org/tutorial/6-viewsets-and-routers" rel="noreferrer">http://www.django-rest-framework.org/tutorial/6-viewsets-and-routers</a></p>
  
  <pre><code>class SnippetViewSet(viewsets.ModelViewSet):
      ...
  
      @link(renderer_classes=[renderers.StaticHTMLRenderer])
      def highlight(self, request, *args, **kwargs):
          snippet = self.get_object()
          return Response(snippet.highlighted)
  </code></pre>
  
  <p>This example add following route:</p>
  
  <pre><code>url(r'^snippets/(?P&lt;pk&gt;[0-9]+)/highlight/$', snippet_highlight, name='snippet-highlight'),
  </code></pre>
  
  <p>It is possible to add an url without pk param, like this?</p>
  
  <pre><code>r'^snippets/highlight/$'
  </code></pre>