Add a Logstash 'V1' Formatter

See https://logstash.jira.com/browse/LOGSTASH-675 for more details. 1.2.0 was released yesterday and, although the documentation doesn't seem to have been updated, does seem to be using this new format.

Basically the 'changes' (from version 0) involve removing the `@` prefix for various fields, and promoting the `extra` and `context` fields to the top level.

I also opted to use the `level_name` rather than `level` in the logs, because it's friendlier to read than a load of numbers.

Thanks.