Input format option (plain, json_event)

Hi,
I've added an input format option to change behaviour of the input line processing. There are two options now 

* plain - old one
* json_event - same as in java logstash

The json_event acts as much as its original logstash alternative - e.g. the hash created for plain  input is merged with line parsed as JSON.

The code has been tested in your production environment and runs fine.

If you like my enhancement, please pull, as I expect more people would appreciate this feature.