Add callbacks

src/callbacks.js

+c3.chart.internal.fn.pushCallback = function(callback){
+    var $$ = this;
+    if(callback){
+        if($$.callbacks){
+            $$.callbacks.push(callback);
+        } else {
+            $$.callbacks = [callback];
+        }
+    }
+};
+
+c3.chart.internal.fn.resolveCallbacks = function(){
+    var $$ = this;
+    if($$.callbacks){
+        $$.callbacks.forEach(function(callback){
+            if(callback){
+                callback();
+            }
+        });
+    }
+    $$.callbacks = [];
+};
+
+c3.chart.internal.fn.resolveDraw = function(options){
+    var $$ = this;
+    $$.resolveCallbacks();
+    $$.updateAndRedraw();
+};
+
+c3.chart.internal.fn.cachedRedraw = function(options, callback){
+    var $$ = this;
+
+    $$.pushCallback(callback);
+
+    if($$.shouldCache){
+        if(!$$.buffer.has("tune-axis")){
+            $$.buffer.onlastfinish("cached-redraw",
+                function(){
+                    $$.resolveDraw(options);
+                }
+            );
+        }
+    } else {
+        $$.resolveDraw(options);
+    }
+};
+
+c3.chart.fn.setDrawCaching = function(should){
+    var $$ = this.internal;
+    $$.config.shouldCache = should;
+};
+
+c3_chart_fn.getDrawCaching = function(){
+    var $$ = this.internal;
+    return $$.config.shouldCache;
+};