Allow to overwrite/customise individual parameters on case class registration

#26 allowed users of the library to "globally" support custom types. This ticket intends to allow even greater deal of customisation - on an individual `case class` parameter (or path to parameter if `case classes` are nested).

## Proposal
Add optional Map parameter to model registration `.register` method , where key is path in case class of the field that user wants to overwrite, and value is Schema. Then use this map during registration, with the highest precedence.