Move markdown plugins config to markdown.plugins

Author: rstacruz

fixture/markdown-plugin-invalid/docpress.json

@@ -1,5 +1,7 @@
 {
   "markdown": {
-    "lalalala": {}
+    "plugins": {
+      "lalalala": {}
+    }
   }
 }

fixture/markdown-plugin/docpress.json

@@ -1,5 +1,7 @@
 {
   "markdown": {
-    "decorate": {}
+    "plugins": {
+      "decorate": {}
+    }
   }
 }

lib/index.js

@@ -273,8 +273,9 @@ function inferTitle (contents) {
 
 var mdCache = {}
 
-function md (plugins, cwd) {
-  const key = JSON.stringify(plugins || {})
+function md (options, cwd) {
+  const plugins = (options && options.plugins) || {}
+  const key = JSON.stringify(plugins)
   if (mdCache[key]) return mdCache[key]
 
   var md = markdownIt({