feat: dag-api direct support

[daviddias]

Hi @wanderer !

As promised, here you go, unixfs-engine riding on top of node.dag. You can use it today with your js-ipfs version by simply:

const node = new IPFS()
node.on('ready', () => {
  const ue = require('ipfs-unixfs-engine')
  const import = new ue.Importer(node.dag)
  // or
  const export = new ue.Exporter(<multihash>, node.dag)
})

[wanderer]

whooooo! thanks @diasdavid