Enabled configuration of open and close icons in indentedTree

Added chart.iconOpen and chart.iconClose in the same format as
chart.tableClass etc that will return the current setting for either
icon value unless an argument is supplied, in which case they will
replace the settng with the new value and return the chart.

src/models/indentedTree.js

@@ -271,6 +271,17 @@ nv.models.indentedTree = function() {
     return chart;
   };
 
+  chart.iconOpen = function(_){
+     if (!arguments.length) return iconOpen;
+    iconOpen = _;
+    return chart;
+  }
+
+  chart.iconClose = function(_){
+     if (!arguments.length) return iconClose;
+    iconClose = _;
+    return chart;
+  }
 
   return chart;
 }