Add command to extract build info from metadata

This will allow to recreate the metadata after patching the FS.

PUBLISHED_FROM=dae228bbf49d71f383e4dc52316cf98aba3914f5

docs/usage-example/items.json

@@ -0,0 +1,5 @@
+{
+  "items": [
+    { "type": "markdown", "name": "intro.md" }
+  ]
+}