[665] CLI accepts custom nuxt.config.js file as argument.

Hi, I think it makes sense to have the cli to accept custom nuxt.config.js as argument.

One scenario is that for test purposes, I need to deploy the site to a CDN for purposes, whose address might look like this:

http://cdn.my-domain.com/test/app-bar/

So in nuxt.config.js, I need to set

router: {
    base: '/test/app-bar/'
},

build: {
    publicPath: 'http://cdn.my-domain.com/test/app-bar/'
}

However, when I need to deploy to the production server, I need to change the nuxt.config.js again.