frontmatter: Move frontmatter content into vm.frontmatter (#551)

Author: petrjoachim

Diff for: src/plugins/front-matter/index.js

@@ -4,7 +4,7 @@ const install = function (hook, vm) {
   hook.beforeEach(content => {
     const {attributes, body} = parser(content)
 
-    Docsify.util.merge(vm.config, attributes.config)
+    vm.frontmatter = attributes
 
     return body
   })