版本：
文件：
- https://cdn.bootcdn.net/ajax/libs/gmaps-marker-clusterer/1.2.2/markerclusterer.js
- https://cdn.bootcdn.net/ajax/libs/gmaps-marker-clusterer/1.2.2/markerclusterer.min.js
The library creates and manages per-zoom-level clusters for large amounts of markers. Google API v3.