Add support for IPv6 hosts and regex filtering

These changes implement the following features:

- IPv6 AAAA host lookups
- Support for multiple ports to be listed on the CLI
- Regex filtering of included hosts
- Only output unique results

If you're happy with the changes in this PR I'll remove any references to "differences" in the README and "Generated by github.com/jinnko/sshfp" in the non-quiet output before merge.