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.
Good point. It should only add those cli options if the `site-id` and `access-tokens` are provided given they are optional.