creating columns from more than one field

[willishq]

We should be able to create a column from more than one field:

$column = $grid->addColumn('name', 'Full Name');
$column->fromFields('first_name', 'last_name');`

This will also require the withFormat() method to change to allow unlimited fields to be passed through if they are available, for instance

$column->withFormat(function ($firstName, $lastName) {
    return "${lastName}, ${firstName}";
});

[willishq]

If there are two from fields, and a format provided which only has one param, should we appy the format to each element?