Add 'figure' tag to the HTML whitelist, closing #41

Closes #41 by including `<figure>` as part of the HTML5 whitelist. 

Before: 

```ruby
>> Loofah.scrub_fragment("<span>hello</span> <figure>asd</figure>", :prune).to_s
=> "<span>hello</span> "
```

After: 

```ruby
>> Loofah.scrub_fragment("<span>hello</span> <figure>asd</figure>", :prune).to_s
=> "<span>hello</span> <figure>asd</figure>"
```