[feature request] additional snapshot naming options

I'd like to have a conf file for making the occasional point in time full snapshots that I might like to use say on other machines.

I tried `snapshot_name` but had to add a `snapshot name` with a hard coded name for each `subvolume` entry to get that to work as I intended.

so I guess I'm asking for some additional naming options
`snapshot_suffix `
`snapshot_prefix `
and while I am asking another option might be
`snapshot_basename`
this would strip any directory from the `subvolume` path when creating the snapshot name/path.

For example

```
snapshot_suffix base # applies to all subsequent subvolumes requested
snapshot_prefix manjaro # applies to all subsequent subvolumes requested
snapshot_basename # applies to all subsequent subvolumes requested
subvolume @shell
subvolume common/@home
subvolume manjaro/@opt
subvolume manjaro/@rootfs
```

would result in

```
manjaro-@shell-base.XXX
majaro-@home-base.XXX
majaro-@opt-base.XXX
manjaro-@rootfs-base.XXX
```

BTW this is an excellent project, well coded and documented. I'm a btrfs noob and it is a great tool.

I'm very good at bash scripts but know nothing about perl but I'm willing to help if only a PR for the updated docs.

No source file or test is named. Start by locating the configuration handling for `snapshot_name`, `subvolume`, and snapshot path construction, then determine how configuration scope applies to subsequent entries. Done means the requested prefix, suffix, and basename options produce the illustrated names and are documented.

Written by the indexing model from the issue text.