Replace hard-coded webpack with node script call

[stanislas-m]

This PR replaces the hardcoded webpack commands (dev & build) with standard package.json scripts which can be called with either npm or yarn.

The new applications will be generated with the following new section in the package.json:

  "scripts": {
    "build": "webpack -p --progress",
    "dev": "webpack --watch"
  }

Buffalo will then call yarn run build (or npm run build) when you call buffalo build; and it will call yarn run dev (or npm run dev) when you call buffalo dev. This allows you to customize the scripts buffalo calls on these steps, providing custom args to the webpack command or even running your own toolchain instead.

Thanks to #1729, we'll preserve compatibility with current apps which didn't migrate by hand or run buffalo fix : if the build or dev scripts can't be found, we'll fallback on the old behavior running the hardcoded webpack commands.

Migration

The buffalo fix command rewrites the package.json file if necessary, adding this new scripts section if it doesn't already exist.

Fixes #1116.