Optional JSON library selection

[icehaunter]

Phoenix and may others now don't use JSON encoding libraries implicitly, instead allowing for configuration via config.exs. This is great, because it allows to use on package across the whole application instead of use multiple variations of the same.

I think this library would benefit from switching to a similar approach, to drop explicit dependency on Poison in favor of config option.

Thanks a lot for a great library!

[mraaroncruz]

It looks like this can be closed from this commit: https://github.com/svetob/logstash-json/commit/81d69a1d429b4eb36c5f3cd7943b5bdcce3ab1cf