Allow prefixing the build label.

It would be nice to prefix the build badge to display multiple build status.

Here are some examples:
![stable build](https://cloud.githubusercontent.com/assets/343404/4177072/86fd2390-3624-11e4-827c-99e9476805de.png)
![dev build](https://cloud.githubusercontent.com/assets/343404/4177073/8a87f648-3624-11e4-84b0-2b96af891368.png)
![upstream build](https://cloud.githubusercontent.com/assets/343404/4177077/0224b768-3625-11e4-8967-18a801ce3984.png)
![1x build](https://cloud.githubusercontent.com/assets/343404/4177079/0730ef4c-3625-11e4-910e-4bb54aaa6e38.png)
![2x build](https://cloud.githubusercontent.com/assets/343404/4177080/09b4bc80-3625-11e4-83f6-d6b2b74143c4.png)