feat: unixfs reify

[tchardin]

This PR does 2 things:

• Reify unixfs directories to select paths via fields selectors and creates a new InterpretAs selector.
• Enable fetching via ipfs path (/path).
• Allow adding directories via the add command.

  Note that it currently doesn't work against go graphsync as it seems to be a bug with the implementation. Investigating with Hannah when she wakes up.

Resolves #38

** Note: adding large directories is very slow at the moment. It seems to be an issue on the unxifs side. Might need to implement our own version...