Allowing to wrap <img> in <figure>

I thought it does the description field justice to only put it in the alt attribute, I tweaked this a little to allow wrapping in a <figure>...

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```