RFE: secure-forward compatible output

It'd be wonderful to have [out_secure_forward](http://docs.fluentd.org/articles/out_secure_forward) compatibility in fluentd-forwarder.

Example use case- with [Docker 1.8](https://docs.docker.com/reference/logging/fluentd/) there is basic Fluent support, but sending the logs off the box requires a forwarder agent, preferably a secure one.