journal: add string method to journal.Priority

[bcicen]

Adds string method to journal Priority level

[lucab]

Thanks for the PR! Do you maybe have a reference to the consumer where you need this? I'm somehow inclined towards not merging this: those constants are mostly mirroring https://golang.org/pkg/log/syslog/#Priority (minus the facility part) and as such they don't have a canonical string representation (which is locale dependent too).