title: DFS visitor does not traverse detached vertices
body: <p>I have a graph represented by adjacency list. I apply depth_first_visit algorithm for it. Everything works almost OK.
  The problem is that the algorithm visits only vertices, which are connected with my starting point vertex. If I have separate vertices (without any connection), they are not traversed.
  Of course I have solved this problem by finding not visited vertices and then starting the algorithm from them but In the documentation it is written that those "detached" vertices shall also be traversed. Question - am I doing something wrong?</p>
  
  <pre><code>typedef boost::adjacency_list&lt;boost::vecS, boost::vecS, boost::directedS, Vertex, Edge &gt; GraphType;
  
  vector&lt;default_color_type&gt; color_map(num_vertices(m_graph));
  
  depth_first_visit(
         m_graph,
         *vp.first,
         custom_dfs_visitor(m_currentPath, visited),
         make_iterator_property_map(
             color_map.begin(),
             get(vertex_index, m_graph),
             color_map[0]),
         Terminator(m_currentPath)
      );
  </code></pre>