Allow ClassMaps to be specified via CsvConfiguration

[IanKemp]

Is your feature request related to a problem? Please describe. Currently, ClassMaps can only be registered for mapping purposes by calling methods on the CsvContext obtained from a reader or writer. This is inconsistent with the rest of the API, which allows up-front configuration via CsvConfiguration.

Describe the solution you'd like Add RegisterClassMap methods to CsvConfiguration, that mimic those available on CsvContext.

[JoshClose]

It actually used to be like that. The configuration is immutable and all mutable properties were moved to the context. This was to fix issues people were having with multi-threading.