Allow use of disabled property of HTML checkboxes

I needed this capability in the project for which I'm using this plugin, but I figured why should it be limited to just me.  The property is defined on the column level, so:
...
```javascript
}, {
                field: 'COMPLETE',
                title: 'Complete',
                align: 'center',
                valign: 'middle',
                sortable: true,
                checkbox: true,
                class: 'info',
                checkboxEnabled: false
            }, {
```
etc.