Support dollar templating in config file

[pspiagicw]

Right now the after-backup commands are written completely by the user.

But we should provide some resolving of the commands using demp.

Let me explain

storeDir = "some-system-path"
after-backup = [
    "tar -czvf backup.tar.gz <the-full-fucking-path>"
]

What if we support dollar templating like we use for groom.
Meaning

storeDir = "some-system-path"
after-backup = [
    "tar -czvf backup.tar.gz $storeDir"
]

It makes it easier for the user to build upon complex commands.
Like

storeDir = "some-system-path"
after-backup = [
    "tar -C $storeDir -czvf backup.tar.gz $storeDir",
    "scp ${storeDir}/backup.tar.gz <some-server>",
# Either delete the `backup.tar.gz` or not, dotback will work.
    "rm ${storeDir}/backup.tar.gz"
]