Feature Request: Add option to customize the delimiter between prefix and timestamp

First of all, thank you for developing such a great tool. btrbk is incredibly useful and I rely on it for my backups.

### Is your feature request related to a problem? Please describe.
I am using btrbk on openmediavault (OMV). OMV has a strict policy regarding the `shadow_copy2` VFS module configuration for Windows Previous Versions support. Specifically, it hardcodes the delimiter to an underscore (`_`) as follows:

```conf
shadow:mountpoint = /srv/dev-disk-by-uuid-xxx/
shadow:snapdir = /srv/dev-disk-by-uuid-xxx/.snapshots/
shadow:basedir = /srv/dev-disk-by-uuid-xxx/sharename/
shadow:sort = desc
shadow:format = _%Y%m%dT%H%M%S
shadow:delimiter = _
shadow:snapprefix = ^sharename\(@hourly\)\{0,1\}\(@daily\)\{0,1\}\(@weekly\)\{0,1\}\(@monthly\)\{0,1\}\(@yearly\)\{0,1\}$
shadow:localtime = no
```

Currently, btrbk uses a dot (`.`) as the default delimiter (e.g., `sharename.20240422T162511`), and there seems to be no option to change this to an underscore.

### Describe the solution you'd like
I would like an option to specify a custom delimiter (e.g., `timestamp_delimiter _`) in the configuration file.

### Describe alternatives you've considered
Currently, I use a custom script to rename snapshots from `sharename.timestamp` to `sharename_timestamp`. After renaming, the Windows "Previous Versions" feature works correctly on OMV. However, having this as a built-in option in btrbk would be much more robust and "native."

### Additional context
In OMV, these settings are automatically generated per share and cannot be modified by the user due to the author's policy and internal logic. Supporting the underscore delimiter would greatly improve compatibility with OMV's Samba implementation.

Thank you for your time and for considering this request!