Prepend or append content to pageSummary

[Sanya1991]

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:

    [
            'attribute' => 'attribute name',
            'filter' => true,
            'pageSummary' => true,
            'pageSummaryPostfix' => ' Example postfix: Ft',
            'pageSummaryPrefix' => 'Example prefix: $ '
        ],

[kartik-v]

Resolved via upgrade to release v2.7.0. The prepend and append options have been added to pageSummaryOptions.