MDL-6368 and MDL-6369 - allow question type plugins to easily provide some bits of...

MDL-6368 and MDL-6369 - allow question type plugins to easily provide some bits of Javascript and CSS.

If there are any of the files styles.css, styles.php, script.js or script.php in the plugin folder, they will be linked to in the head of the quiz attempt page (only for the question types used on that page).

If you need to do something more complicated in your question type, you can override the get_html_head_contributions method instead.