Modularize sbt-cake

[by-dam]

sbt-cake shouldn't depend on Gatling, ...

It's not something that should be imposed on the users as their project might not even use Gatling.
But it turns out that Gatling is needed by the CakeTestRunnerPlugin so we should make things more modular for a better user experience.

(Same thing goes for docker ...)

[fommil]

and wartremover / scalastyle and a bunch of other stuff that not every project wants.

[Slakah]

Completely agree. A clear example would be the sbt docker-compose wrapper, which could easily be definitely be useful to a lot of projects external to cake.

[fommil]

I have some improvements for that in my project, as I kept working on it.