Add custom messages

[kzielonka]

In my current project validation shouldn't just return messages but objects with fields like :error_code (for example INVALID_EMAIL_ERROR and :message (which is an normal message like Email format is invalid.).
I didn't know how to do it using :yaml or :i18n messages providers so I created this pull request.

I extended Dry::Validation::Schema to accept custom messages object.
The setting option messages can now be one of :yaml, :i18n or :custom.
If :custom option is set custom_messages should also be set as Object which implements #default_locale and #[](key, options) methods.
Method #[](key, options) should return objects which implement #% (for example Strings but not necessary) or is a subclass of Dry::Validation::Messages::Abstract (please check tests for more details how it can be sued).

Plese check is it ok and I hope that it will be merged into master ;)

[dsounded]

just before ?

[solnic]

This API was moved to dry-schema project. It's possible that its specialization will be re-added to dry-validation, for now this PR is outdated. We can get back to this when things are more settled in master, as the lib was rewritten on top of dry-schema.