[552] Consider splitting webpack & related in to a new npm module

When nuxt.js is running in production, it doesn't need webpack. However, because it's listed as dependency it will always be installed.

If an npm module were made (I propose nuxt-dev) which contains these dependencies, a user of Nuxt would be able to add that module to their devDependencies list. The resulting package.json for a Nuxt project would look like this:

{
  "name": "my-app",
  "dependencies": {
    "nuxt": "latest"
  },
  "devDependencies": {
    "nuxt-dev": "latest"
  },
  "scripts": {
    "dev": "nuxt",
    "build": "nuxt build",
    "start": "nuxt start"
  }
}

The nuxt-dev package could contain everything needed "build" the application, while the nuxt package would contain the "runtime" dependencies and any runtime-level code.