'Simple' extensions

[ciaranmcnulty]

I think we should enumerate the common extension points in behat, which I suspect are:

1. Register a Formatter
2. Do something else while listening to hooks
3. Provide objects for auto wiring into context

And then provide a simple interface (or >1 interface) that extensions can implement to achieve these. this would remove a lot of the extension-writing overhead of having to grok the symfony container

We could pick one of the use cases above and go with it (with a candidate extension)

I think we would support the existing mechanism still, for any extensions that needed to get deep into Behat's guts