Remove flexible label support from Rack middleware #111

Supporting flexible labels in our out-the-box Rack middleware commits us to maintaining what is currently quite a confusing API.

@dmagliola discussed a few ways to make the API less weird, but they always resulted in the middleware accepting multiple lambdas for custom behaviour as arguments and having almost no behaviour provided out the box - sort of defeating the purpose!

We can always come back and add a better implementation of this functionality later, but it will be a pain to take this version of it away.

I’m in favour of only supporting our fixed set of labels in the Rack middleware we provide, and having a README section advising people to do their own thing if they want something more sophisticated.