More domain name systems

There should be support for more domain name systems. Some of them are more decentralised, some of them are less, but it would still be good if more would be supported.

Ideally, each system should have it's own plugin (to provide support to disable only one), but some of them may share some code. All of them should be enabled by default (to reach more users) but it should be possible to disable them.

This are some ideas I have:

- [x] [NameCoin](https://namecoin.org/) (#31)
- [ ] [Classic DNS (with OpenNIC)](https://www.opennic.org/) (#104)
- [ ] [Ethereum Name Service](https://ens.domains/)
- [ ] [Tor Onion Domains](https://en.wikipedia.org/wiki/.onion)
- [ ] [EmerDNS](https://emercoin.com/en/emerdns) (#142)
- [ ] [Handshake](https://handshake.org/)
- [ ] [Unstoppable Domains](https://unstoppabledomains.com/)
- [ ] [OneName](https://onename.com/)
- [ ] ZeroNet ID providers (#1696)

The issue proposes support for several domain name systems, including Classic DNS, Ethereum Name Service, Tor Onion Domains, EmerDNS, Handshake, Unstoppable Domains, OneName, and ZeroNet ID providers, but names no implementation files or tests. First narrow the work to one system and find the existing NameCoin support and plugin entry points; done should include isolated enable/disable support and coverage for the selected system.

索引モデルが issue の本文から書いたものです。