Implement ECS-Compatibility Mode

[yaauie]

As a part of the effort to make plugins able to run in an ECS-Compatible manner
by default in an upcoming release of Logstash, this plugin needs to implement
an ECS-Compatibility mode that does not implicitly use fields that conflict
with ECS.

Absent target directive, this plugin populates fields verbatim from the
JSON-encoded payload it is given, and may benefit from requiring an explicit
target when run in ECS-Compatibility mode, or otherwise reduce the risk of collision.

[kares]

delivered in #49