allow passing only volume name to export/import

[Guiorgy]

file name will be inferred as the volume name with .tar.gz as file extension

E.g.

$ ./vackup export mosquitto_config
mosquitto_config
./
./mosquitto.conf
Successfully tar'ed volume mosquitto_config into file mosquitto_config.tar.gz
$ ls
mosquitto_config.tar.gz  vackup
$ ./vackup import mosquitto_config
mosquitto_config
./
./mosquitto.conf
Successfully unpacked mosquitto_config.tar.gz into volume mosquitto_config

PS. I haven't updated the usage text.

[BretFisher]

Thanks for all the PRs. I'm going to close this one as I feel with the 4 commands always requiring from -> to, it gets confusing if some of those don't require a to.

[Guiorgy]

Sure, that's also a valid choice.