add sass to vuecli code example

Example 1: vue js sass

# /!\ if you work with vue@2.6.12, it seems sass-loader@11.0.0 doesn't work
# So, in terminal:
yarn add node-sass sass-loader@10.1.1

Example 2: vue add sass

<!--
# Run this command in the root dir of your vue project:
npm i sass-loader node-sass style-loader
-->

<!-- And later in your vue component -->
<style lang="sass">
/* Your sass code */
</style>