fix($markdown): "index" is dropped in a link with "*index" pattern

Reported at #996

Author: ulivz

packages/@vuepress/markdown/lib/link.js

@@ -2,7 +2,7 @@
 // 1. adding target="_blank" to external links
 // 2. converting internal links to <router-link>
 
-const indexRE = /(.*)(index|readme).md(#?.*)$/i
+const indexRE = /(^|.*\/)(index|readme).md(#?.*)$/i
 
 module.exports = (md, externalAttrs) => {
   let hasOpenRouterLink = false