Support JSON front matter as used by the Hugo site generator (fixes #270).

test/hugo-quickstart-example-json.md

@@ -0,0 +1,13 @@
+{
+    "title": "Another Hugo Post",
+    "description": "Nothing special, but one post is boring.",
+    "date": "2014-09-02",
+    "categories": [ "example", "configuration" ],
+    "tags": [
+        "example",
+        "hugo",
+        "toml"
+    ],
+}
+
+# Heading {MD025}