fix(gridOptions): use this.gridOptions in bind fn

using the first arg in the bind method looks at the parent context, which might not be the view-model with the `gridOptions`. For example in a repeat.for, the view-model properties are not available, which can be wrong if aurelia slickgrid in a repeat.for. Using `this.gridOptions` assures us we are using the options that are bound

aurelia-slickgrid/src/aurelia-slickgrid/aurelia-slickgrid.ts

@@ -214,7 +214,7 @@ export class AureliaSlickgridCustomElement {
    */
   bind(binding: any, contexts: any) {
     // get the grid options (priority is Global Options first, then user option which could overwrite the Global options)
-    this.gridOptions = { ...GlobalGridOptions, ...binding.gridOptions };
+    this.gridOptions = { ...GlobalGridOptions, ...this.gridOptions };
 
     // Wrap each editor class in the Factory resolver so consumers of this library can use
     // dependency injection. Aurelia will resolve all dependencies when we pass the container