Support for cat'ing and get'ing subtrees.

[daviddias]

unixfs-engine needs to support:

• export('/ipfs/QmHash', ipldResolver)
• export('/ipfs/QmHash/a', ipldResolver)
• export('/ipfs/QmHash/b.txt', ipldResolver)
• export('/ipfs/QmHash/a/b/c.txt', ipldResolver)
• export('QmHash/a/b/c.txt', ipldResolver)
• export('QmHash/a/b/c.txt', ipldResolver)
• It also needs to support a subtree that starts on a non protobuf node, but the path leads to the start of a file that is a protobuf.

This is blocking ipfs/js-ipfs#875

[daviddias]

Happening on #175. Track there -> #175