Implement `bak off -k`

[ChanceNCounter]

bak off has a --keepers argument stubbed (which, in clap terms, technically means it's implemented, but not hooked up, and so it's hidden.)

In the original bak, this would, although clumsily, allow a user to specify one or more bakfiles to retain when calling bak off. This is probably desirable functionality and should be brought back.

edit: bak off should get this functionality bak, but bak down doesn't need it. In the original bak, bak down would automatically erase bakfiles, so you could deal with it in one step. I have rethought this, and now believe bak down should instead offer a flag along the lines of --clear to erase bakfiles, and otherwise retain them unless and until the user does a corresponding bak off.