Add support for custom column in admin.

[cjhaas]

These changes allow custom columns to be displayed in the backend for groups using WordPress filters. To be clear, it doesn't add support to set these values, it just exposes filters for other plugins to add their own columns and value.

Usage:

//Add a new column that is not sortable
add_filter(
            'groups_admin_columns',
            function( $column_display_names )
            {
                return array_merge(
                                    $column_display_names,
                                    array(
                                            'test' => array( 'column_display_name' => 'Test' )
                                        )
                                );
            }
        );

//Render the custom column
add_filter(
            'groups_admin_columns_cell_html_contents_test',
            function( $cell_html_contents, $result )
            {
                return 'Custom test for group ' . $result->group_id;
            },
            10,
            2
        );

//Set the column's CSS classes
add_filter(
            'groups_admin_columns_cell_html_classes_test',
            function( $cell_html_classes, $result )
            {
                return array( 'test-column' );
            },
            10,
            2
        );

//Optionally change the cell's wrapper tag. No real good reason but consistent with other changes
add_filter(
            'groups_admin_columns_cell_html_tag_test',
            function( $cell_html_tag, $result )
            {
                return 'th';
            },
            10,
            2
        );