permit class assignment on links in reports

It's possible to assign a link url & a hover text to a cell in a report. This minor pr  adds the ability to add a class. I don't love it, but there is precedent for this format in the way _hover & _link are handled & in the Overview.tpl

Note the usecase is to add the crm-summary-link class to a contact - allowing for the link to display in a hover-box rather than needing to click on it, per search result handling

```

$url = CRM_Utils_System::url('civicrm/profile/view',
'reset=1&snippet=4&gid=' . $this->summaryOverlayProfileId . '&id=' . $row['civicrm_contact_1'],
$this->_absoluteUrl
);
$rows[$rowNum]['civicrm_contact_1_link'] = $url;
$rows[$rowNum]['civicrm_contact_1_hover'] = ts("View contact details for this record.");
$rows[$rowNum]['civicrm_contact_1_class'] ="crm-summary-link";

```