Simplify process of registering handlers and customizing default handler

[gmazzap]

At the moment Wonolg ships a default handler that needs to be configured.

It has a lot of configurations, and should probably be simplified.

Moreover, entirely replace with a custom handler is quite complex as well.

The whole thing should probably be reconsidered taking into account possibility to expose more the Monolog registry.

[gmazzap]

Refactoring happen, let's move forward.