add {subtree: "retain"} flag to allow skipping diff from app space

archive/v0.1.3/change-log.html

@@ -65,6 +65,7 @@
 <ul>
 <li>diff no longer touch the DOM when processing <code>style</code> attributes and event handlers</li>
 <li>returning a thennable to a resolution callback in <code>m.deferred().promise</code> now causes the promise to adopt its state </li>
+<li>diff now correctly clears subtree if null or undefined is passed as a node</li>
 </ul>
 <hr>
 <p><a href="/mithril/archive/v0.1.2">v0.1.2</a> - maintenance</p>