Adding the possibility to inactive image drag and drop

Adding the possibility to users configure if drag and drop is active or not.

```javascript
$('element').mediumInsert({
  editor: mediumEditor,
  addons: {
    images: {
      useDragAndDrop: false
    }
  }
});
```

I'm not sure about setting in image tag draggable as false and setting onmousedown to return false.