feat: add option merge to command create (#4828)

packages/@vue/cli/bin/vue.js

@@ -63,6 +63,7 @@ program
   .option('-g, --git [message]', 'Force git initialization with initial commit message')
   .option('-n, --no-git', 'Skip git initialization')
   .option('-f, --force', 'Overwrite target directory if it exists')
+  .option('--merge', 'Merge target directory if it exists')
   .option('-c, --clone', 'Use git clone when fetching remote preset')
   .option('-x, --proxy', 'Use specified proxy when creating project')
   .option('-b, --bare', 'Scaffold project without beginner instructions')

packages/@vue/cli/lib/create.js

@@ -29,7 +29,7 @@ async function create (projectName, options) {
     exit(1)
   }
 
-  if (fs.existsSync(targetDir)) {
+  if (fs.existsSync(targetDir) && !options.merge) {
     if (options.force) {
       await fs.remove(targetDir)
     } else {