Add custom logical types

[jwtryg]

closes #435

Hi @nrwiersma

This PR adds the possibility to register custom logical types for the schema. These logicaltypes are primitive, and does not include logic for handling custom encoding/decoding, but simply adds the the custom logicalType to be referenced in the schemas.

I have added the WithCustomLogicalType function as a SchemaOption, but not at all schemas use this option - I saw this pattern was already used for other schema options.

Let me know what you think :)