Add Clack.Middleware.Let and replace Clack.Middleware.Stdout by it.
Add Clack.Middleware.Backtrace as a default middleware. It is a kind of "safety net" for production environment.

By this changes, the debug options of handlers are needless. But I leave them for now.
Fix a bug of Clack.Builder which was contained in f9172d. `builder' couldn't treat NIL in a list of middlewares.