Support overriding --group-by args via env var/config

[SteVwonder]

Love the project! I am trying to use it across multiple hosts but with a single B2 bucket. It works great as long as all of the hosts have different paths, but when the paths are the same across multiple hosts, the retention policy breaks down.

Any objection to keeping paths,tags as the default but opening up the args to being configured via env var (maybe RESTIC_BACKUP_GROUP_BY)? Then people can override the group-by options if they want (e.g., I would use paths,tags,host).

If that is amenable to you, I'd be happy to open a PR with the change. Thanks again for the awesome tool.

[SteVwonder]

For now, I've just added a tag with the hostname via RESTIC_BACKUP_TAG which as far as I can tell results in the same outcome retention-wise.