Implement the jsipfs dns command

[daviddias]

Following: https://github.com/ipfs/js-ipfs/pull/925/files

We want to support the same command that go-ipfs supports: jsipfs dns.

From go-ipfs:

» ipfs dns --help
USAGE
  ipfs dns <domain-name> - Resolve DNS links.

SYNOPSIS
  ipfs dns [--recursive | -r] [--] <domain-name>

ARGUMENTS

  <domain-name> - The domain-name name to resolve.

OPTIONS

  -r, --recursive bool - Resolve until the result is not a DNS link. Default: false.

DESCRIPTION

  Multihashes are hard to remember, but domain names are usually easy to
  remember.  To create memorable aliases for multihashes, DNS TXT
  records can point to other DNS links, IPFS objects, IPNS keys, etc.
  This command resolves those links to the referenced object.

  For example, with this DNS TXT record:

        > dig +short TXT _dnslink.ipfs.io
        dnslink=/ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy

  The resolver will give:

        > ipfs dns ipfs.io
        /ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy

  The resolver can recursively resolve:

        > dig +short TXT recursive.ipfs.io
        dnslink=/ipns/ipfs.io
        > ipfs dns -r recursive.ipfs.io
        /ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy

[daviddias]

@hacdias shipped this one with #1172, thank you! ❤️