fix: glob patterns error on windows (vuejs#348)

Author: meteorlxy

lib/dev.js

@@ -26,12 +26,14 @@ module.exports = async function dev (sourceDir, cliOptions = {}) {
       console.error(chalk.red(err.stack))
     })
   }
-
+  console.log(path.join(sourceDir, '**/*.md'))
+  console.log(path.resolve(sourceDir, '**/*.md'))
   // watch add/remove of files
   const pagesWatcher = chokidar.watch([
-    path.join(sourceDir, '**/*.md'),
-    path.join(sourceDir, '.vuepress/components/**/*.vue')
+    '**/*.md',
+    '.vuepress/components/**/*.vue'
   ], {
+    cwd: sourceDir,
     ignored: '.vuepress/**/*.md',
     ignoreInitial: true
   })