Fix #105

* configured tags can access request.attributes with simple expression vars (id)
* response is no longer available as a value for expressions; but there’s no use case
  for that anyway
* update docs
well, you may want to have some stuff related to the query string. Imagine you have a query string param allowing to embed the full relations in the JSON output of your API rather than just the related ids. When using this feature, some more content is used to generate the output, requiring to invalidate the request for this URL when the related data changes while there is no need in case the root object only is displayed.
Being able to add extra tags based on the fact that the query string uses this param would be nice. And this requires accessing the Request, not only the attributes