Fix string & null formatter, add CLI option to use null formatter

The first commit fixes the definitions of `string_formatter` and `null_formatter`, and the second one makes `null_formatter` available via the CLI using `-F raw`.

My reason for wanting to bypass formatting is that I am already producing logstash-compliant JSON events in a couple of application(s), and I just want to use beaver to ship the log files reliably.

Let me know if you'd prefer two separate PRs.