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.
@cantino Use of `interpolated` is almost always a call without an argument, so its context needs to be held as a state of an Agent instance, and the state of a run has be saved in a thread local storage for thread safety which some Agents might expect.  So, it seemed to me that the use of `Thread.current[]` was a logical conclusion to be safe.

But I know your feeling, and I might be too wary.  I hid where to store as implementation details, so if it should be enough to just use an instance variable instead, we could just change the `interpolation_context` accessors as such.