Respect :fmt-output-opts by using the system's output instead of hand-rolling it

The rotor  appender was not respecting :fmt-output-opts because it was hand-rolling its own formatting instead of using the output that is already handed to it, with stacktraces already pre-formatted. Rotor was dumping unreadable stacktraces that looked like this:

https://www.refheap.com/44858

This patch is required in order for options passed to the stacktrace via :fmt-output-opts to work, and so that configs like this will work:

```clojure
  (log/set-config! [:appenders :rotor] (assoc rotor/rotor-appender
                                          :fmt-output-opts {:nofonts? true}))
```