Add zIndex support for marker

1) Allow user to pass 'zIndex' and 'optimized' props to marker class. Setting the 'optimized' property to 'false' seems to help the user determine which marker displays on top using the zIndex. See: https://stackoverflow.com/questions/11845916/google-maps-marker-zindex-doesnt-work-for-two-icon-types-symbol-and-string

Author: paabrown

dist/components/Marker.js

@@ -136,7 +136,9 @@
             icon = _props.icon,
             label = _props.label,
             draggable = _props.draggable,
-            title = _props.title;
+            title = _props.title,
+            optimized = _props.optimized,
+            zIndex = _props.zIndex;
 
         if (!google) {
           return null;
@@ -153,7 +155,9 @@
           icon: icon,
           label: label,
           title: title,
-          draggable: draggable
+          draggable: draggable,
+          optimized: optimized,
+          zIndex: zIndex
         };
         this.marker = new google.maps.Marker(pref);