added feature to add weights to the heatmap (#572)

src/google_heatmap.js

@@ -1,9 +1,10 @@
 export const generateHeatmap = (instance, { positions }) =>
   new instance.visualization.HeatmapLayer({
     data: positions.reduce(
-      (acc, { lat, lng }) => {
+      (acc, { lat, lng, weight = 1 }) => {
         acc.push({
           location: new instance.LatLng(lat, lng),
+          weight,
         });
         return acc;
       },