Add style enforcement rules

Add rules to enforce ATX-closed headers, dashes for unordered list elements,
and `1.` for ordered list elements.

.mdl_config.json

@@ -1,4 +1,10 @@
 {
+  "MD003": {
+    "style": "atx_closed"
+  },
+  "MD004": {
+    "style": "dash"
+  },
   "MD013": {
     "code_blocks": false,
     "tables": false
@@ -6,5 +12,8 @@
   "MD024": {
     "allow_different_nesting": true
   },
+  "MD029": {
+    "style": "one"
+  },
   "default": true
 }