support orderby meta_value_*

[salkhwlani]

add support pass orderby meta_value_* in site_sortables & admin_cols

'site_sortables' => array(
    'meta_name' => array(
        'meta_key' => 'meta_name',
        'orderby' => 'meta_value_num',
        'default' => 'desc',
    )
),

'admin_cols' => array(
    // A meta field column:
    'meta_name' => array(
        'title'    => 'meta title',
        'meta_key' => 'meta_name',
        'orderby' => 'meta_value_num',
        'default' => 'desc',
    ),
),

for more information about meta_value_* look section “Order & Orderby Parameters”

[johnbillion]

Thanks for the patch, but this means clicking on the column header could potentially not sort by that field, which is not what any user would expect. The purpose of clicking a column header is to sort by the values show in the column.