Allow configuration of the Middleware stack

Move the functionality of `Lotus::Middleware` from the application to a
configuration block to allow users to configure the middleware stack.
When the application is loaded, `Lotus::Middleware#load!` will still get
called to load the final middleware stack into the Rack::Builder app
that wraps the router.

Closes #27.

Signed-off-by: David Celis <me@davidcel.is>
Agreed on line 14; the other will be a little ugly since middlewares take the form of a [Class, *args, &block] array. But I can verify that none of the members of `middleware.stack` have `Rack::Static` as the first element.