Disable checkBox through a top-level data field

Example:
```
var isOddRow = function(index) {
  return (index % 2) === 0;
};
dataSet = dataSet.map(function (rec, index) {
    return {
        user: {
            firstName: rec.firstName,
            lastName: rec.lastName,
        },
        disableCheckbox: isOddRow(index)
    };
});

$scope.grid = {
    data: dataSet,
    columnDefs: [
        new ngReactGridCheckbox($scope.selections, {disableCheckboxField: 'disableCheckbox'}),
        {
            field: "user.firstName",
            displayName: "First Name",
        },
        {
            field: "user.lastName",
            displayName: "Last Name"
        }
    ]
};

```

'disableCheckboxField' is a ngReactGridCheckbox option that reference a data (dataset in this example) property/field to determine whether the checkbox for the associated row is disabled or not.