title: Making a node in Tkinter Treeview unselectable
body: <p>I would like to know , how to make certain nodes in a Treeview object un-selectable, i.e. when clicked on such a node,the node shouldn't highlight.</p>
  
  <p>I am working on<br>
  Python 3.3/2.7<br>
  Tkinter</p>