Add functionality for customizing derived Haskell type names

[jmickelin]

Introduces a third way to handle Avro namespaces, namely by providing your own function to map namespaced Avro types to Haskell types.

This provides a middle ground between IgnoreNamespaces (doesn't handle name clashes) and HandleNamespaces (very very verbose), allowing you to do for example:

disambiguateOffendingType :: Text -> [Text] -> Text
disambiguateOffendingType t ns
  | ns <> [t] == ["some", "namespace", "somewhere" "OffendingType"] = "OffendingType"
  | ns <> [t] == ["some", "other", "namespace", "using", "the", "same", "type", "name", "for", "whatever", "reason", "OffendingType"] = "OffendingTypeOther"
  | otherwise = t

Used like so:

deriveAvroWithOptions defaultOptions { namespaceBehavior = Custom disambiguateOffendingType } "my-avro-file.avsc"

[AlexeyRaga]

Thanks!