Added section for PRs that are not closed but that you have approved.

This checks for the thumbs up github emoji to indicate approval.

![trailerthumbs](https://cloud.githubusercontent.com/assets/45890/3268463/5a176f1e-f2dd-11e3-858c-e218a3f7ca27.png)

I'm just submitting this PR upstream in case you're interested. It's very handy for our workflow. I might spend some time making it optional, and/or allowing users to change the substring match that indicates approval. But don't count on it... :)