Pino: Support injecting serializers via .child()

[mcollina]

Currently it is not possible to add serializers in child loggers.

See https://github.com/mcollina/pino/pull/57 as an example of how it could be implemented.

[dgonzalez]

Quick question, should a serializer added to the child replace the existing ones from the parent or should it be added to the existing ones? (as far as I am aware it is an array). I might be able to work on this today/weekend.

[mcollina]

it should be added to the existing ones, but just for that child logger, replacing the old one if it's there.

[dgonzalez]

Perfecto. I'll try work on something today.