Add BitBalloon support

Add support for [BitBalloon](https://www.bitballoon.com/), a static-site hosting service. 

**Backstory**: I have a few customers who are using other means for building static sites and deploying to BitBalloon; I wanted to recommend they use Travis for this type of scenario. I'd like to contribute BitBalloon support to dpl with the hopes of making it available in travis-ci.org and travis-ci.com.
According to [the docs](https://github.com/BitBalloon/bitballoon-ruby/blob/master/README.md) for the CLI, `access_token` and `site_id` are only necessary if there *isn't* a .bitballon file.

Could you update the docs to take that into account?