`plakar config` to handle configuration of importer origins, exporter targets and repositories

[poolpOrg]

We need a git-like plakar config subcommand to configure the credentials of the various endpoints we connect to:

• importers
• exporters
• repositories

Come up with a yaml structure, read at startup, allow substituting on the command line, ie:

$ plakar config foobar.username=gilles
$ plakar config foobar.password=foobarbazqux
$ plakar config foobar.location=s3://localhost:9000

$ plakar on @foobar backup /private/etc