Add %app and %pid patterns for syslog appenders.

When present (including by default) the tokens `%app` and `%pid` will be replaced with the name of the Dropwizard application and PID of the process respectively.

This is in line with the convention for many Linux processes to prefix their log messages with: "ApplicationName[PID]: ".

The PID can only be determined automatically for some UNIX JVMs (including HotSpot). For others, it will be omitted entirely.