The username and hostname for new keys can be specified with the new
--user and --host flags, respectively. The flags are used only by the
`key add` command and are otherwise ignored.
This allows adding keys with for a desired user and host without having
to run restic as that particular user on that particular host, making
automated key management easier.
Co-authored-by: James TD Smith <ahktenzero@mohorovi.cc>
