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>
I defined this for convenience, but I could also see people not really using `Lotus::Middleware#[]`, so I can remove it if there's agreement there.