TODO

Presuming tarsnap doesn't already take care of this, a lock file
should be used to prevent the same config file to be processed
by multiple instances at the same time.

Allow *_exec commands to create a temporary directory that can be backed;
currently this is problematic because you cannot specify a dynamically
created directory in the "source" option.

Support alternative names per backup job. This allows a graceful rename
of a backup job.

Support globbing when specifying sources.

Support includes, for example also "include /var/lib/vz/*/backup.conf"
to load the backup files for all virtual machines.

A way to expire a set of backups, without explicitely specifying each
individual job. I.e., say a server creates the backups, but the
delete-permission key is on a different machine. That second machine
wouldn't need a copy of the config file (or many config files, if includes
are used), and instead could expire all backup sets with a single command,
like: tarsnapper expire --regex "$name-$date".

Multiple manually specified dateformat values may make sense, if
needing to parse a set of already existing archives.