Allow index.md files as index pages (#23)

Until now, only `README.md` files got treated as a directory index.
This commit also allows `index.md` to be used as index files.

lib/prepare.js

@@ -189,7 +189,7 @@ async function genComponentRegistrationFile ({ sourceDir }) {
   return `import Vue from 'vue'\n` + components.map(genImport).join('\n')
 }
 
-const indexRE = /\breadme\.md$/i
+const indexRE = /\b(index|readme)\.md$/i
 const extRE = /\.(vue|md)$/
 
 function fileToPath (file) {