add brushExtent to chart to enable the ability to set default focus range.

There is a brushExtent when init, so I guess maybe you guys just forgot to put the public function available.

src/models/lineWithFocusChart.js

@@ -1,4 +1,3 @@
-
 nv.models.lineWithFocusChart = function() {
 
   //============================================================
@@ -552,6 +551,12 @@ nv.models.lineWithFocusChart = function() {
     y2Axis.tickFormat(_);
     return chart;
   };
+  
+  chart.brushExtent = function(_) {
+    if (!arguments.length) return brushExtent;
+    brushExtent = _;
+    return chart;
+  };
 
   //============================================================