feature: enable cat and get files from sub dag trees

[daviddias]

This has been a low-hanging fruit that we've opened for contribution for a while now and now with the upgrade to CIDs and new DAG API, it might not be as low hanging fruit as it was before.

Ref:

• track: Files API + MFS #60
• ipfs.files.cat path ipfs-inactive/interface-js-ipfs-core#138
• ipfs.files.get and ipfs.files.cat do not support ipfs-path format #837

Tasks:

• Update interface-ipfs-core to have tests specifically for this feat: add support for paths on get and cat ipfs-inactive/interface-js-ipfs-core#143
• Update docs to describe this feature feat: add support for paths on get and cat ipfs-inactive/interface-js-ipfs-core#143
• Update js-ipfs-api to support this Feat/update iic ipfs-inactive/js-ipfs-http-client#559
• Update js-ipfs to support this (this PR)
  • Level up unixfs-engine to support this feature Support for cat'ing and get'ing subtrees. ipfs-inactive/js-ipfs-unixfs-engine#174

[daviddias]

Still getting errors with the new ipfs-unixfs-engine //cc @pgte

  1) interface-ipfs-core tests .files callback API .cat with ipfs path:
     AssertionError: expected [Error: Non-base58 character] to not exist
      at ipfs.files.cat (node_modules/interface-ipfs-core/src/files.js:258:32)
      at pull.collect (src/core/components/files.js:90:20)
      at node_modules/pull-stream/sinks/reduce.js:10:5
      at node_modules/pull-stream/sinks/drain.js:20:24
      at /Users/koruza/code/js-ipfs-unixfs-engine/node_modules/pull-stream/sources/error.js:5:5
      at next (node_modules/pull-stream/sinks/drain.js:16:11)
      at sink (node_modules/pull-stream/sinks/drain.js:37:9)
      at pull (node_modules/pull-stream/pull.js:41:14)
      at Function.cat.promisify (src/core/components/files.js:86:7)
      at Object.cat (node_modules/promisify-es6/index.js:34:27)
      at Context.it (node_modules/interface-ipfs-core/src/files.js:257:22)

  2) interface-ipfs-core tests .files callback API .cat with ipfs path, nested value:
     Error: Timeout of 80000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.


  3) interface-ipfs-core tests .files callback API .get with ipfs path, nested value:
     Uncaught TypeError: Cannot read property 'pipe' of undefined
      at DestroyableTransform.stream.pipe.through.obj [as _transform] (node_modules/interface-ipfs-core/src/files.js:446:33)
      at DestroyableTransform.Transform._read (node_modules/through2/node_modules/readable-stream/lib/_stream_transform.js:159:10)

[daviddias]

@pgte excellent work! Just tested again locally and confirmed it works flawlessly :D