Add msg about possible existing file overwrite

[whyvez]

I ran the cli over an existing folder with an index.html file and it was overwritten. Would be nice if there was a warning about this when the cli is invoked.

[zigomir]

@whyvez vue-cli will prompt you with ? Target directory exists. Continue? (Y/n) question if you're trying to init a project into existing directory. Also, if you're trying to init it into current directory it will prompt you with: ? Generate project in current directory? (Y/n).