An optional output location for the showing.

A minor convenience but avoids a situation where usethis show ... > file.txt will create file.txt initially empty which influences the behaviour of usethis show on presence of the file, e.g. for usethis show sonarqube with sonar-project.properties.

If --output-file is set without specifying a file name, in some cases we should use a sensible default, e.g. in the sonarqube case above.