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.
Just noticed this - the description says that the default is a single tab character, but it looks like the default is an empty string.