Prepend or append content to pageSummary

DataColumn class: Two new attribute: pageSummaryPrefix and pageSummaryPostfix.
ColumnTrait class: renderPageSummaryCellContent method modified. 

I modified ColumnTrait renderPageSummaryCellContent method, i add pageSummaryPrefix before content and pageSummaryPostfix after content.

Use in view: 
```php
    [
            'attribute' => 'attribute name',
            'filter' => true,
            'pageSummary' => true,
            'pageSummaryPostfix' => ' Example postfix: Ft',
            'pageSummaryPrefix' => 'Example prefix: $ '
        ],
```