An example of a custom CMS menu-item created using a DataExtension

config.yml

LeftAndMain:
  extensions:
    - CustomMenuItemExtension

Custom-CMS-Menu-Item-Extension.php

<?php

class CustomMenuItemExtension extends LeftAndMainExtension {

    public function init() {
        // unique identifier for this item. Will have an ID of Menu-$ID
        $id = 'LinkToGoogle';

        // your 'nice' title
        $title = 'Google';

        // the link you want to item to go to
        $link = 'http://google.com';

        // priority controls the ordering of the link in the CMS menu
        $priority = -3;

        // Add your own attributes onto the link
        $attributes = array(
            'target' => '_blank'
        );

        CMSMenu::add_link($id, $title, $link, $priority, $attributes);
    }
}