feat: folder creation (#696)

packages/core/dev-test/index.html

@@ -14,11 +14,7 @@
             'lobby.jpg': {
               content: '',
             },
-          },
-        },
-        _posts: {
-          assets: {
-            uploads: {
+            'Other Pics': {
               'moby-dick.jpg': {
                 content: '',
               },
@@ -27,6 +23,8 @@
               },
             },
           },
+        },
+        _posts: {
           '2015-02-14-this-is-a-post.md': {
             content:
               '---\ntitle: This is a YAML front matter post\ndraft: true\nimage: /assets/uploads/moby-dick.jpg\ndate: 2015-02-14T00:00:00.000Z\n---\n\n# I Am a Title in Markdown\n\nHello, world\n\n* One Thing\n* Another Thing\n* A Third Thing\n',