[feature][ZFS] Snapshots ought to be ignored (or hidable)

[keltia]

By default with ZFS pools, duf shows snapshots as FS which it should probably not do. I'd either add a "snapshot" type to the -hide command or just ignore them by default (could maybe even be used by btrfs as well, but I don't know where snaphots work there).

╭──────────────────────────────────────────────────────────────────────────────╮
│ 1819 local devices                                                           │
├───────────────────┬──────┬─────────┬───────┬───────┬────────┬────────────────┤
│ MOUNTED ON        │ SIZE │    USED │ AVAIL │  USE% │ TYPE   │ FILESYSTEM     │
├───────────────────┼──────┼─────────┼───────┼───────┼────────┼────────────────┤
│ /                 │ 1.2T │ 1021.2M │  1.2T │  0.1% │ zfs    │ tank/root      │
│ //.zfs/snapshot/2 │ 1.2T │  378.1M │  1.2T │  0.0% │ zfs    │ tank/root@2013 │
│ 013071801         │      │         │       │       │        │ 071801         │
│ //.zfs/snapshot/2 │ 1.2T │    1.1G │  1.2T │  0.1% │ zfs    │ tank/root@2019 │
│ 0191031           │      │         │       │       │        │ 1031           │

1819 local devices includes all snapshots.

Thanks for duf, I like it :)