Add nxlog parser

See issue #9

I switched `SOURCE_HOSTNAME` to `SOURCE-HOSTNAME` to respect old school DNS conventions disallowing underscores (which logstash grok patterns [observe](https://github.com/elasticsearch/logstash/blob/master/patterns/grok-patterns#L27)).
I do like this idea, but this approach is more of a tagging sort of thing and logstash only supports tagging under the `tags` field. We'd need some sort of alternate implementation.

Alternatively, we just keep everything in `tags` and just do `parsed-syslog_standard`, `parsefail-nginx_combined-grok`. I think I'd like it better if it goes `{rule}-{filter}(-{line/clarifier})?` - then we're able to segment by all parse failures, parse failures for content types, parse failures for content types by filter.