Added JSON formatter, field name identifers and type casting option.

 * "format.type = string|json|protobuf|avro" config option
 * field name identifiers %{@NAME}x (for JSON,etc encoding)
 * type-casting to !options (!num supported)

Change-Id: Ia5d61c0a3be01722d8191d0ea4a8010445530ff9
Added 'output = null' for testing purposes.

Change-Id: If5f1e2fe89acf5caa60c21a3c4b1b88723097fa6