Allow configuration of stackTracePattern via SyslogAppenderFactory

I came across an issue using syslog whereby tabs `\t` were escaped with `#011` in our logs. Considering this is an option configurable on the appender, it'd be useful to expose it via `SyslogAppenderFactory`.

In my particular case, the ops guys weren't keen to change rsyslog config because of the risk of impacting other existing apps. It's also conceivable that some deployments may want to use other patterns for stack traces.

I've changed the docs to reflect the additional configuration parameter.