feat(exporter): implement recursive file export

implement recursive file export and fixed a bug where
the data in intermediate nodes was not getting output.

src/exporters/file.js

@@ -1,32 +1,34 @@
 'use strict'
 
+const traverse = require('pull-traverse')
 const UnixFS = require('ipfs-unixfs')
 const pull = require('pull-stream')
 
 // Logic to export a single (possibly chunked) unixfs file.
 module.exports = (node, name, ds) => {
-  const file = UnixFS.unmarshal(node.data)
-  let content
+  function getData (node) {
+    try {
+      const file = UnixFS.unmarshal(node.data)
+      return file.data || new Buffer(0)
+    } catch (err) {
+      throw new Error('Failed to unmarshal node')
+    }
+  }
 
-  if (node.links.length === 0) {
-    content = pull.values([file.data])
-  } else {
-    content = pull(
+  function visitor (node) {
+    return pull(
       pull.values(node.links),
       pull.map((link) => ds.getStream(link.hash)),
-      pull.flatten(),
-      pull.map((node) => {
-        try {
-          const ex = UnixFS.unmarshal(node.data)
-          return ex.data
-        } catch (err) {
-          console.error(node)
-          throw new Error('Failed to unmarshal node')
-        }
-      })
+      pull.flatten()
     )
   }
 
+  let content = pull(
+    traverse.depthFirst(node, visitor),
+    pull.map(getData)
+  )
+
+  const file = UnixFS.unmarshal(node.data)
   return pull.values([{
     content: content,
     path: name,