Custom Context Menu Items

It would be nice to be able to have custom context menu items. 

Perhaps something like an "extraContextMenuItems" option.

```javascript
var options = { 
  ...
  extraContextMenuItems: [
    {
     text: "Custom Item",
     title: "Custom Title",
     click: (e)=>console.log("clicked", e),
     ...
    }
  ]
}
```