Restricted direct access to template files

[xecdev]

This PR fixes: #18.

To test:

• Install the updated plugin
• Try to access the template files directly like: https://test.com/wp-content/plugins/wordpress-plugin/templates/admin/content.php
• You should see a blank page

[Klakurka]

Actually I don't get a blank page - I get a 404:

[xecdev]

Yes, that's expected behavior. A 404 or a blank page.