fix($dev): using config.yml/toml doesn't reload changes (close: #520)

lib/dev.js

@@ -44,8 +44,13 @@ module.exports = async function dev (sourceDir, cliOptions = {}) {
 
   // watch config file
   const configWatcher = chokidar.watch([
-    path.join(sourceDir, '.vuepress/config.js')
-  ], { ignoreInitial: true })
+    '.vuepress/config.js',
+    '.vuepress/config.yml',
+    '.vuepress/config.toml'
+  ], {
+    cwd: sourceDir,
+    ignoreInitial: true
+  })
   configWatcher.on('change', update)
 
   // also listen for frontmatter changes from markdown files