Use of @metadata in 2.0

Based on https://github.com/logstash-plugins/logstash-output-elasticsearch/issues/218, and other similar requests, I propose that we start putting much of the metadata in inputs into the `@metadata` field in Logstash 2.0. This may allow for a much cleaner output for many users.

Additionally, we could have a flag (within the input block, perhaps) that, if set, puts those fields into the event, as with 1.x.

Doing this could also pave the way to using `@metadata` more for 2.0 plugins, which is a good thing, I think.