Add ability to customize JsonFactory and JsonGenerator.

Decorate JsonFactory when the formatter starts.

Also delay parsing of the customFields until after the formatter starts,
to ensure JsonFactory is properly decorated first.

Simplify LogstashMarker interface by just using the ObjectCodec (ObjectMapper) from the JsonGenerator,
rather than passing a separate ObjectMapper instance.

Unit tests and documentation for decoration.
Merge branch 'json_factory_access'