Custom naming format for snapshots

Currently my snapshots are names like this using `timestamp_format long-iso`:
```
ID 548 gen 26991 top level 265 path snapshots/mhr2/home/home.20220707T050000+0200
ID 549 gen 26991 top level 265 path snapshots/bch2/root/root.20220707T050000+0200
ID 554 gen 23569 top level 265 path snapshots/desk/data/data.20220707T050000+0200
```
I would like to have `snapshots/desk/data/20220707T050000+0200` only.
Either you could add support for a custom naming scheme but that could have issues when changing the scheme later and btrbk would not find the subvolumes anymore. Or you could just add a switch to have a long or a short format.

I guess the snapshot name is constructed from variables `subvolume` and `snapshot_dir` or `target send-receive`:
```
volume /mnt/btrfs-top-lvl/
# ...
snapshot_dir snapshots/root
# ...
target send-receive /mnt/usb/mobiledata/snapshots/bch2/root
```