registerCommand (name, opts, fn) {
    if (typeof opts === 'function') {
      fn = opts
      opts = null
    }
    this.service.commands[name] = { fn, opts: opts || {} }
  }
Register a command that will become available as `vue-cli-service [name]`.

@param {string} name
@param {object} [opts]
  {
    description: string,
    usage: string,
    options: { [string]: string }
  }
@param {function} fn
  (args: { [string]: string }, rawArgs: string[]) => ?Promise