feat: add option merge to command create

[zyy7259]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information:

• Usage: vue create --merge .
• This option works like the --force option.

[sodatea]

I don't feel like adding this feature. Because the end result is still unpredictable and the directory could still get messed up.
A warning on the overriding might suffice.

[zyy7259]

What I want to do with this option is auto generating projects by some tools based on vue-cli.

The tool I used needs to write some files before calling vue-cli.

I just want to skip the prompt, and merge files immediately.

[pksunkara]

I agree with @sodatea here. If you want to, you can create a new folder and merge those 2 folders with some other tools locally.

[zyy7259]

By default, without any cli options, if the target dir is existing, @vue/cli will prompts the user to either delete the target folder or merge with it.

And there is an option --force to skip the prompt and delete the target folder. So I just want to add an option --merge to skip the prompt and merge with the target folder.

Actually I'm writing a tool which combined @vue/cli with a custom package manager. I need to write some info into the package.json in the target folder first. And during the creation of the target project, these info will be used by the custom package manager to installing deps.

So I just want this --merge option to skip the prompt and automate the whole creation procedure.

[pksunkara]

That actually sounds better. If you are okay, we can merge this @sodatea

[pksunkara]

This is still failing. You think there's an issue with your code?

FAIL packages/@vue/cli/__tests__/Upgrader.spec.js

[pksunkara]

Never mind, it has been happening on dev too