Strip contents of <script> tags.

This will turn this:

```ruby
SanitizationFilter.call('<script>JavaScript!</script>').to_s #=> JavaScript!
```

into this: 

```ruby
SanitizationFilter.call('<script>JavaScript!</script>').to_s #=> 
```

Dunno if this would be useful for everyone, but for my purposes there aren't any cases where I want to show what is inside a script tag.