Support DNS 0x20

Basically make *everything* case-insensitive.

```toml
[groups.blocklist]
type      = "static-responder"
answer    = [ "IN A 0.0.0.0" ]

[groups.filter]
type               = "blocklist-v2"
blocklist-resolver = "blocklist"
blocklist-format   = "domain"
blocklist          = [ "example.com" ]
```

```bash
$ dig +short example.com
0.0.0.0
```

```bash
$ dig +short Example.com
23.192.228.80
```
