feat(google-map): add get and set methods to Marker class

fixes #798

src/plugins/googlemap.ts

@@ -486,6 +486,21 @@ export class GoogleMapsMarker {
     );
   }
 
+  /**
+   * Gets a value
+   * @param key
+   */
+  @CordovaInstance({sync: true})
+  get(key: string): any { return; }
+
+  /**
+   * Sets a value
+   * @param key
+   * @param value
+   */
+  @CordovaInstance({sync: true})
+  set(key: string, value: any): void { }
+
   @CordovaInstance({ sync: true })
   isVisible(): boolean { return; }