Give access to response status and header values in WebsiteAgent [PATCH]

This is a PR for #445, adds a variable `_response_` for interpolation in WebsiteAgent.
How about `add_response_to_interpolation_context!`

and then as a method

```ruby
def add_response_to_interpolation_context!
  locals = {
    '_response_' => ResponseDrop.new(response)
  }
  if event = Thread.current[:current_event]
    set_interpolation_context(event.to_liquid(locals))
  else
    set_interpolation_context(EmptyDrop.new(locals))
  end
end
```

But maybe that's no cleaner.