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
what if we just add the request for advanced users, mention it in the doc but do not have any examples using the request, to keep them simple? the strategy to overwrite a possible `request` attribute with the Request means that there would be a BC break if we would add this later.