Add the ability to load middleware outside Plack::Middleware::Debug::*

It sucks that if you add some non-generic Middleware to your own
codebase that it needs to live in the Plack::Middleware::Debug::*
namespace. This patch trivially adjusts the prepare_app() loader so
we'll support +-prefixed middleware names like Plack::Builder.
Merge pull request #18 from avar/avar/support-+-prefixed-middleware

Add the ability to load middleware outside Plack::Middleware::Debug::*