Add sprintf ability to room_id field in hipchat output

We had a design requirement to be able to notify different rooms based on logstash inputs. This PR allows us to dynamically send exceptions to different hipchat rooms based off of tag values:

    hipchat {
      token   => "12345"
      room_id => "%{env}-%{pod}"
    }

I was unable to run the spec tests on my local mac, but I will try another run from another host soon.