Expose zoom speed option (#19)

* Expose zoom speed option

* Avoid unnecessary operations

src/litegraph.js

@@ -5180,6 +5180,7 @@ LGraphNode.prototype.executeAction = function(action)
 
         this.ds = new DragAndScale();
         this.zoom_modify_alpha = true; //otherwise it generates ugly patterns when scaling down too much
+        this.zoom_speed = 1.1 // in range (1.01, 2.5). Less than 1 will invert the zoom direction
 
         this.title_text_font = "" + LiteGraph.NODE_TEXT_SIZE + "px Arial";
         this.inner_text_font =
@@ -6899,9 +6900,9 @@ LGraphNode.prototype.executeAction = function(action)
         var scale = this.ds.scale;
 
         if (delta > 0) {
-            scale *= 1.1;
+            scale *= this.zoom_speed;
         } else if (delta < 0) {
-            scale *= 1 / 1.1;
+            scale *= 1 / this.zoom_speed;
         }
 
         //this.setZoom( scale, [ e.clientX, e.clientY ] );