[11] Allow custom port # through npm config

First off, great work so far on nuxt.js

For this issue, some documentation here: https://docs.npmjs.com/files/package.json#config

Ideally, the package.json would then look like this:

{
  "scripts": {
    "start": "nuxt"
  },
  "config": {
    "nuxt": {
      "port": 4000
    }
  }
}

This is critical for folks who want to host multiple nuxt.js apps/sites on one server.