[WIP] Hipchat message tempates

The feature will allow setting message templates from the .drone.yml Hipchat block. Implementing #272.

- [ ] Use hipchat message templates (started, success, failure) from .drone.yml
- [ ] Parse input strings safely with fallback to default templates
- [ ] Use go templates http://golang.org/pkg/text/template/
- [ ] Add hipchat tests
- [ ] Add all available values to HipchatContext