Library support

[szuecs]

First, thanks for creating this!
I think it would be beneficial for a lot of other issues like #65 or #98 and likely others to change the project to library first design.
It would help to do zalando/skipper#2215 and from my experience with skipper it's enabling a lot of things that you never thought before. The longer you wait the more work it will be, so start early by moving your main module to cmd/<binary-name>/*.Go