Add a flag to use ngrok

[ElYaiko]

Hello
It would be good to start the module only if a flag is passed when the dev is started, example:
npm run dev --ngrok

So won't be necessary to comment/uncomment the module everytime Ngrok won't be used, this could be added as an option to the config:

ngrok: {
  byCmdFlag: true,
}

[karladler]

You can add the condition using env variables:

 modules: [ 
    // ...
    process.env.NUXT_ENV_NGROK === 'true' && '@nuxtjs/ngrok', 
]