Add ZeroNet support

[filips123]

Add support for ZeroNet site address. ZeroNet uses old-style Bitcoin addresses as site addresses.

Also see multiformats/multicodec#140. This PR depends on multiformats/js-multicodec#48.

[filips123]

@pldespaigne PR for adding ZeroNet to multicodec package was merged and released. I created tests and updated multicodec version to correct one. I also updated demo with better support for onion, onion3 and zeronet. Can you now merge this PR?

I also linked my Python implementation from README and your package from my package. This is because I think that it would be good to notify developers about other implementations of the content hash in different languages.

[pldespaigne]

Hi @filips123 ! I don't have much time right now, but I will look at your PR before Sunday

[filips123]

@pldespaigne Can you now check this?

[Gudahtt]

There is a conflict with README.md. Could you update this PR @filips123 ?

[pldespaigne]

I'm closing this PR because since v2.4.4, all multicodec are being encoded to utf-8 as a fallback