Add option to output unparsed line in message field, in addition to parsing

[GrahamHannington]

For background to this request, please see my related question in the Logstash category of the Elastic discussion forum: "Map timestamp using first time value in incoming JSON Lines?".

It might also be useful to have this option in the json codec, for use with a file input, rather than, say, a tcp input. In which case: guess I'd need to open a separate-but-corresponding issue for that codec, right?