Issue #708: Add '--force' flag to 'vue init' command

[codetheorist]

• Add mock-stdin package as a dependency
• Add mock-stdin to ask.js file
• Add --force flag in bin/vue-init
• Add conditional to send newline character at each prompt of --force flag is set

[posva]

force is probably not the right naming choice. Something like --default (next version) or --yes seems more appropriate

[codetheorist]

Now you've mentioned it, it does seem more logical @posva for it to be called default or yes.

[yyx990803]

We will no longer be releasing new features to the 2.x branch, but thanks for contribution.