Show up warnings on providing unknown commands

[James George]

As per now vue-init just has an init which initializes the project.
It is good to have suitable warnings thrown in case the user provides unknown commands

Usage: vue-native [options] [command] <command>

  Options:

    -v, --version                 output the version number
    -h, --help                    output usage information

  Commands:

    init [options] <projectName>
  
  Unknown command <command>.