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>
Merge branch 'config-middleware' of https://github.com/davidcelis/lotus into davidcelis-config-middleware
Merge branch 'davidcelis-config-middleware'
Refactoring for Lotus::Configuration#middleware, this also includes fixes and test improvements