Allowing to wrap <img> in <figure>

I found it useful to allow wrapping the img in a figure tag and add a figcaption, based on the description you enter in the popup - makes a little better use of it than just putting it in the alt attribute imo.
In the init call, add ```uploadimage_figure: true``` to create this structure:

```html
<figure class='figure'>
  <img src='..' alt='whatever>
  <figcaption class='figcaption'>whatever</figcaption>
</figure>
```

You can also pass other classes with ```uploadimage_figure_class``` and ```uploadimage_figcaption_class```