use include function instead of get_template to fix inaccessible variables with template customization

While the widget currently searches for a custom template file in the theme folder (using `get_template` ), when it is found the custom template fails to render properly because the variables such as `$instance` are not available when the template is loaded via Wordpress'  `get_template_part` method.

This fix uses the `include` function instead, it is a solution based on from [kdev's blog](http://keithdevon.com/passing-variables-to-get_template_part-in-wordpress/) - this fix is tested and working with a custom template in the theme's `views/kebo-twitter-list.php` folder.