Force checkboxes

A new variable for questions to force checkboxes even if only one answer is
true. For if you **don't** want to give the user a hint that only one answer is true. Maybe he then selects 2 checkboxes although only one answer is true. With radios this was not possible with force_checkbox this now is possible.

Set the variable true in your quizJSON for every question you want to force checkboxes
```javascript
...
"force_checkbox": true,
...
```