Added three configuration properties to SyslogAppender

Added three configuration properties to SyslogAppender: facility, stacktrace.pattern, and throwable.excluded. These provide more flexibility for segregation of log output for different audiences and systems, such as for creating audit-only logs, structured logs, and logs intended for debugging application errors.
Do you think it makes sense to hardcode this list? If we're going to make this configurable, maybe we should just make it free text and then uppercase it. I'm assuming that the configurer editing this knows what they are doing.