Document and improve support for webpack

[fgrehm]

Something simliar to what is documented for bourbon here vuejs/vue-loader#95 (comment) that doesn't require us to know in advance where files are located. Right now I have to:

var pathToVital = path.resolve(__dirname, '../node_modules/vital-css/sass')

module.exports = {
  // ...webpack stuff...
  sassLoader: {
    includePaths: [pathToVital]
  }
}

But it'd be nice if we were able to just:

module.exports = {
  // ...webpack stuff...
  sassLoader: {
    includePaths: [require('vital-sass').includePaths]
  }
}