Add directory list command (files.ls)

[haadcode]

Add files.ls(hash) command so that added directories can be listed.

[daviddias]

@haadcode has mentioned on the last sprint meeting that this is no longer a priority, postponing to the Milestone of Full Feature parity

[haadcode]

No, cat with offset isn't a priority anymore (it never was, miscommunication in Lisbon).

This would be nice to have asap. I need it to display directories in Orbit (works currently only in the Electron app). However, this can be postponed after interop has been done because I can't use it anyways in Orbit until js-ipfs can fetch the added directories from go-ipfs.

[daviddias]

let's track this one in the Files tracking issue: #60