fix to ensure middleware is always applied

When another plugin (such as livereload) has already defined a route, then the `express.router` middleware gets "automagically" inserted in that location in the middleware stack, and anything that's added later via `app.use` is basically ignored when a matching route is found. 

This fix changes how minicms adds the middleware to ensure that it always runs before any minicms route.

(It would be nice if Docpad split `serverExtend` into two different events, one for middleware and one for routes. But, as it stands, this is what we have to do. There is a `serverBefore` event, but it is triggered before the `server` is even created so it isn't much use here.)