Fix export date format; add EXPORT_ prefix in example; add md

- Without the EXPORT_ prefix, those properties were as good as not
  present; only the default values were exported.
- Hugo does not like slashes in dates; fix that
- Also add the resultant .md file

example-site/content-org/construct-hugo-front-matter-from-menu-meta-data.org

@@ -4,9 +4,9 @@
 * Menu Meta Data in YAML Front Matter
 :PROPERTIES:
 :EXPORT_FILE_NAME: menu-meta-data-yaml
-:EXPORT_DATE: 2017/07/17
-:HUGO_MENU_NAME: main
-:HUGO_MENU_WEIGHT: 100
-:HUGO_MENU_PARENT: posts
+:EXPORT_DATE: 2017-07-17
+:EXPORT_HUGO_MENU_NAME: main
+:EXPORT_HUGO_MENU_WEIGHT: 100
+:EXPORT_HUGO_MENU_PARENT: posts
 :END:
 Testing the addition of /menu/ meta data to the front matter.

example-site/content/posts/menu-meta-data-yaml.md

@@ -0,0 +1,13 @@
+---
+title : "Menu Meta Data in YAML Front Matter"
+date : 2017-07-17
+tags : []
+draft : false
+menu :
+  main:
+    identifier : "menu-meta-data-in-yaml-front-matter"
+    parent : "posts"
+    weight : 100
+---
+
+Testing the addition of *menu* meta data to the front matter.